Part 8

A study of the development of _Scolopendra_ has shown that the antennae of the adult are the appendages of the second postoral metamere and the mandibles those of the fourth, the first postoral metamere, which has a pair of transient preantennal appendages, and the third, which has no appendages, being excalated at an early stage of embryonic growth. Furthermore, behind the legs of the last pair two pairs of appendages are present. The second of these persists as the gonopods of the adult, but the first is suppressed. Possibly, however, it is represented in the male of _Scutigera_ by the anterior branches of the gonopods. The cerebral or cephalic portion of the nervous system consists of a quadrilobate mass. From the two upper lobes, which are set transversely, arise the ocular nerves; from the two lower lobes, which are united by a transverse commissure, spring the antennal nerves in front and the chords which form the oesophageal collar behind. These chords unite below the oesophagus to form the compound suboesophageal ganglion, whence the nerves for the four pairs of jaws arise. The ventral system consists of a double chord uniting in each of the leg-bearing segments in a ganglionic swelling which gives off four pairs of nerves to the limbs and tissues of the somite. There is a single ganglion in the genital segment.

[Illustration: Modified from Heymons, _Bib. Zool._, 1901, by permission of E. Nagele.

FIG. 1.

A, Diagram of anterior extremity of an early embryo of _Scolopendra_, ventral view; cl, clypeus; lb, labrum; m, mouth; p.a, preantennal appendage; a, antenna; int, premandibular rudiment; mdl, mandible; mx, maxilla; p.g, palpognath; t.g, toxicognath; lg. 1, first pair of walking legs.

B, Posterior end of a later embryo of _Scolopendra_, ventral view, showing the anal segment or telson (t); the legs of the last pair in the adult (lg. 21) and the two rudimentary pairs of legs (lg. 22, lg. 23).]

Eyes are frequently absent. When present they may be either simple or compound, i.e. consisting externally of a single lens (monomeniscous) of or an aggregation of lenses (polymeniscous). Simple eyes vary in number on each side of the head from one, as in _Henicops_, to many as forty, as in some species of _Lithobius_. In _Scolopendra_, where there are four, the corneal lens is a biconvex thickening of the cuticle. The soft or retinal portion of the eye beneath the lens consists of an aggregation of large cells forming a single layer continuous with the epidermic cells of the circumocular area. Thus the eye is monostichous. The arrangement of the cells, however, is peculiar. They are invaginated to form what may be described as a very deep cup with exceedingly thick walls and correspondingly narrow median space, the outer surface of the cup being formed by the inner or proximal ends of the cells and the inner surface by their outer or distal ends. It results from this arrangement that the cells forming all but the bottom of the invagination lie horizontally, i.e. at right angles to the vertical axis of the eye. From the distal ends of the cells are secreted chitinous rhabdomeres, forming a rhabdom which occupies and fills up the central portion of the cup beneath the middle of the corneal lens. The outer ends of the cells are nucleated and are continuous with the fibres of the optic nerve, which passes from the outer surface of the bottom of the cup to the brain. Compound eyes are found only in the _Scutigeridae_. Externally the eye consists of one hundred or more little lenses or lenticles. The retinal portion is composed of a corresponding number of ocular units or ommatidia. Each ommatidium is an elongated cone with its broad extremity abutting against the corneal lenticle. It consists of a non-nucleated crystalline cone developed from embryonic cells, and is enveloped in three tiers of large nucleated cells. The cells of the outermost tier are heavily pigmented; those of the middle and innermost (proximal) tiers, the retinal cells, are at their inner extremities produced into threads continuous with the fibres of the optic nerve. In the space between these cells and the crystalline cone which they surround, there is a layer of rhabdomeres deposited apparently by the cells.

[Illustration: A and B after Heymons, _Bibl Zool_, 1901, by permission of E. Nagele.

C after Adensamer, _Verh. z. b. Verein_, Vienna, 1893, pl. vii.

FIG. 2.

A, Brain of _Scolopendra_. n.ant, Antennal nerves; n.opt, ocular nerves; n.pr.ant, preantennal nerves; oes. comm, oesophageal commissure.

B, Section of Eye of Scolopendra. len, Corneal lens; ret, retinal or visual cells; n.opt, optic nerve.

C, Ocular unit or ommatidium of compound Eye of _Scutigera_. len, corneal lenticle; c.c, crystalline cone; 1, pigmented cells of outermost tier; 2, 3, retinular cells of middle and innermost tiers; rbd, rhabdomeres; n.opt, optic nerve; pg, pigment cells.]

The alimentary canal is a simple tube running without convolutions from the mouth to the anus. Its anterior portion or pharynx, which arises from the stomodaeal invagination in the embryo, is short; a pair of large, so-called salivary glands open into it. The mesenteric part of the canal is relatively wide and receives at its junction with the hind-gut the excretory products of a pair of very long and slender malpighian tubes of proctodaeal origin. The posterior end of the canal, arising from the proctodaeum, is relatively short and narrow.

[Illustration: FIG. 3.--Diagram of Alimentary Canal of _Lithobius_.

a, Anus.

mg, Mid-Gut.

hg, Hind-Gut.

mt, Malphighian tubule.

s.gl, Salivary gland.

lg. 1, lg. 15, Legs of first and fifteenth pairs.]

The generative organs vary in structural details in different centipedes. In the male of _Lithobius_ the testes consist of a single coiled tube lying above the alimentary canal. The slender vas deferens which proceeds from its hinder end divides posteriorly into a right and left branch, embracing the gut and uniting beneath it to form a common chamber or atrium within the genital orifice. The atrium receives the secretion of two pairs of large accessory glands; and a pair of tubes, or vesiculae seminales, open, one on each side, into the divided sperm ducts close to their point of origin above the intestine. The organs of the female are very similar. There is a large median ovary followed by a short oviduct forming a circum-intestinal collar and a common atrium. Into the latter open a pair of short receptacula seminis and the slender duct of two pairs of large accessory glands. There is nothing in the female corresponding to the supra-intestinal vesiculae seminales of the male. In the male of _Scolopendra_, on the contrary, there are as many as twelve pairs of somewhat sausage-shaped testes, approximated two by two. From each pair proceed two slender ducts which open into a median duct coiled in the posterior third of the body and much expanded in the last three of the leg-bearing segments. The right and left portions of the intestinal ring of the genital duct are unequally developed, and there are no vesiculae seminales, but two pairs of accessory glands communicate with the genital atrium as in _Lithobius_. In the female _Scolopendra_ the right and left portions of the intestinal collar are also unequally developed, and only a single pair of accessory glands besides the receptacula seminis open into the atrium.

[Illustration: After Heymons, _Bibl. Zool_, 1901, by permission of E. Nagele.

FIG. 4.--Posterior portion of generative organs of male of _Scolopendra_ (A), of female (B). t, Testes; v.d, vas deferens; ov, ovary; _r.s_, receptaculum seminis; gl. acc, accessory glands; g.o, generative orifice.]

The heart is tubular and lies in the middle dorsal line immediately beneath the integument. It consists of a series of chambers corresponding roughly to the leg-bearing segments, and lies in a blood-sinus formed by a pericardial membrane whence large alary muscles extend to the sides of the body. Each chamber gives off in _Scolopendra_ a pair of fine lateral vessels, and is furnished at its posterior extremity with a pair of orifices by which the blood re-enters the organ from the pericardial space. From the anterior chamber, which lies in the first or second leg-bearing segment, proceed three arteries, a median which runs forwards into the head to supply the brain and other organs, and a lateral which with its fellow of the opposite side forms an oesophageal aortic collar. From the sides of the latter arise vessels to the gnathites, and from its inferior portion an unpaired vessel passes forwards into the head and another backwards above the nerve chord to the posterior end of the body, supplying each segment in its course with a delicate lateral branch. In _Scolopendra_ the chambers of the heart, excepting the first and last, which are small, are subequal in size; but in forms like _Scutigera_ where the terga are very unequal in size a corresponding inequality in the size of the chambers is manifested.

[Illustration: A after Newport, _Phil. Trans._, 1843. B after Haase, _Zool. Beitrage_, i. pt. 65, 1884, by permission of J.N. Kern. C after Haase, loc. cit.

FIG. 5.

A, Anterior extremity of _Scolopendra_, showing two chambers of the heart (h), the aortic ring (a), the alae cordis (a.m) and a cardiac orifice (o).

B, Two segments of _Scolopendra_, showing the branching and anastomosing tracheae and a spiracle (sp).

C, A pair of tufted tracheae of _Scutigera_. d, Dorsal plate; t.s, tracheal sac; tr, tracheal tubes.]

In all centipedes, except _Scutigera_, respiration is effected by chitinized tracheal tubes which extend with their ramifications throughout the body and open to the exterior by means of spiracles perforating the lateral or pleural membrane of more or fewer of the somites below the edge of the terga. Spiracles are never present upon the anal, genital and last leg-bearing somites, and only rarely, as in _Henicops_, upon the somite bearing the legs of the first pair. In the majority of cases the spiracles are circular, sigmoid or slit-like orifices, with chitinized rim, leading into a pocket-like integumental infolding, from which emanate numerous small tracheal tubes which soon anastomose to form the main tracheal trunks. In _Dacetum_, one of the _Scolopendridae_, there is no pocket-like infolding, the small tracheal tubes opening direct to the exterior on a large subcircular plate where their apertures fuse to form a complicated network. The apertures, as in the case of other genera, are protected by fine hairs; and the tracheae themselves are strengthened by a fine spiral filament. In the _Lithobiidae_ the tracheae do not anastomose; but in _Scolopendra_ and _Geophilus_ the main trunks in each segment fuse transversely with those of the opposite side and also longitudinally with those of the preceding and succeeding segments.

In _Scutigera_ the tracheae differ both in structure and position from those of all other Chilopoda. The spiracles, unpaired and seven in number, open in the median dorsal line. Each leads into a short sac from which five tracheal tubes depend into the pericardial blood-sinus.

Existing Chilopoda may be classified as follows, into five orders referable to two subclasses--

Subclass I. Pleurostigma. Order 1 Geophilomorpha. Order 2 Scolopendromorpha. Order 3 Craterostigmomorpha. Order 4 Lithobiomorpha.

Subclass II. Notostigma. Order 5 Scutigeromorpha.

SUBCLASS I, PLEUROSTIGMA.--Chilopods furnished with a rich system of branching tracheal tubes, the spiracles of which are paired and open upon the pleural area of more or fewer of the somites. Each leg-bearing somite contains a distinct tergum and sternum, the number of sterna never exceeding that of the terga. Eyes are either preserved or lost; when preserved they are represented either by a single one-lensed ocellus or by an aggregation of such ocelli on each side of the head. The anterior portion of the head, bearing the labrum, is bent sharply downwards and backwards beneath the larger posterior portion lying behind the antennae, so that these appendages, approximated in the middle line, project directly forwards from the margin of the head formed by this retroversion of the labral area. The maxillae are short and have no sensory organ; the palpognaths consist of four segments, and the toxicognaths have their basal segments fused to form a single coxal plate.

_Order 1. Geophilomorpha_.--Chilopods with a large and indefinite number of somites, most of which are partially or completely divided into a smaller anterior segment, represented by a pretergal and two presternal sclerites, and a larger posterior segment bearing the spiracles and legs. Spiracles are present upon all the leg-bearing somites except the first and last; and the legs which are short and subequal in length consist of six segments, the basal of which remains small. There are no eyes, and the antennae consist invariably of fourteen segments. The tergal plate of the somite bearing the toxicognaths always remains distinct and separates the head-shield from the tergum of the first leg-bearing somite. The penultimate and antepenultimate segments of the toxicognaths are reduced on the preaxial side of the appendage to the condition of arthrodial integumental folds and suppressed on the postaxial side where the distal segment or fang is firmly jointed to the femoral segment. In the last leg-bearing somite the pleural sclerites coalesce with the coxa of the appendage; but the second segment (trochanter) of this appendage does not fuse with the third (femur). The genital and anal somites are not retractile within the last leg-bearing somite, and the gonopods typically persist in the male as small two-jointed appendages and in the female as jointed or unjointed sclerites. The young are hatched with the full number of segments.

[Illustration: FIG. 6. (After Latzel, _Die Myr. öst.-ung. Mon._ vol. i. "Chilopoda," Vienna, 1880.)

A, Upper view of anterior extremity in _Geophilus_.

a, Basal segments of antennae.

c, Cephalic plate.

t.palp, Tergal plate of somite, bearing palpognaths.

t.tox, Tergal plate of somite, bearing toxicognaths (_tox_).

t.lg.1, Tergal plate of somite, bearing legs of first pair.

B, Toxicognaths of _Scolopendra_, showing the large coxal plate and the reduced penultimate and antepenultimate segments.

C, Terminal segment or fang of the same, showing the orifice of the poison gland.]

_Remarks._--The Geophilomorpha are universally distributed in suitable localities. The number of families into which the order should be divided is as yet unsettled, some authors admitting several groups of this rank, others referring all the genera to a single family, _Geophilidae_. In habits the _Geophilidae_ are mostly subterranean, living in the earth and feeding principally upon earthworms. Occasionally they may be found eating fruit or fungi, probably for the sake of moisture. Although without eyes, they are extremely sensitive to light, and when exposed to it crawl away in serpentine fashion to the nearest sheltered spot, feeling the way with their antennae. They can, however, progress with almost equal facility backwards, using the legs of the posterior pair as feelers. Differing from the majority of the family in habits are the two species _Linotaenia maritima_ and _Schendyla submarina_, which live under stones or seaweed between tide-marks on the coasts of western Europe. Most, if not all, the species are provided with glands, which open upon the sterna and secrete a fluid which in some forms (_Himantarium_) is blood-red, while in others it is phosphorescent. In the tropical form _Orphnaeus phosphoreus_ the fluid is known to possess this property; and its luminosity has been repeatedly observed in England in the autumn in the case of _Linotaenia acuminata_ and _L. crassipes_.

The number of pairs of legs within this family varies from between thirty and forty to over one hundred and seventy. Corresponding discrepancies are observable in size, the smallest specimens being less than 1 in. long and barely 1 mm. wide, while the largest example recorded, a specimen of _Notiphilides_ from Venezuela, was 11 in. long and 1/3 of an inch wide.

When pairing takes place the female fertilizes herself by taking up a spermatophore which a male has left upon a sheet of web for that purpose. The female lays a cluster of eggs in some sheltered spot, sometimes in a specially prepared nest, and encircling them with her body, keeps guard until the young disperse and shift for themselves.

[Illustration: FIG. 7.--_Scolopendra morsitans_ (after Buffon).

A, a, Cephalic plate.

b, Tergum of segment, bearing first pair of legs (d).

c, Tip of palpognath.

e, Antenna.

f, Toxicognath.

g, Last pair of appendages, enlarged and directed backwards.]

_Order 2. Scolopendromorpha._--Chilopods differing principally from the Geophilomorpha in that the number of leg-bearing somites is definitely fixed at twenty-three or twenty-one. These are differentiated into larger and smaller, which alternate with nearly complete regularity. The anterior portion of each somite is only partially cut off as a subsegment. The tergal plate of the somite bearing the toxicognaths is suppressed, probably by fusion with the tergum of the first leg-bearing somite. The antennae consist of a number of segments varying from seventeen to about thirty, and usually differing in the individuals of a species. The second segment (trochanter) of the legs of the last pair is coalesced with the third (femur). In only one genus, namely _Plutonium_, which occurs in Italy, is there a pair of spiracles for each leg-bearing segment, except the first and last, as in the Geophilomorpha. In most genera there are only nine pairs of spiracles situated upon the 3rd, 5th, 8th, 10th, 12th, 14th, 16th, 18th and 20th leg-bearing segments, as in _Scolopendra_, _Cormocephalus_, _Cryptops_, &c. In genera with twenty-three pairs of legs, like _Scolopocryptops_, there is an additional pair of spiracles on the twenty-second pedigerous segment; and a few genera such as _Rhysida, Edentistoma_, possess a pair upon the 7th segment. Eyes, when present, are always four in number on each side. The newly hatched young has the full complement of appendages.

This order is divided into four families:--_Scolopendridae_ (_Scolopendra_, _Rhysida_), _Cryptopidae_ (_Cryptops_, _Theatops_), _Scolopocryptopidae_ (_Scolopocryptops_, _Otocryptops_) and _Newportudae_ (_Newportia_). Apart from the frigid zones it is cosmopolitan in distribution, though only one genus (_Cryptops_) extends into north temperate latitudes. In the tropics and warmer countries of the southern hemisphere the genera and species are particularly abundant, and individuals reach the greatest dimensions, some specimens of the tropical American species _Scolopendra gigantea_ exceeding 12 in. in length. They are strictly carnivorous, their diet consisting of any animal, vertebrate or invertebrate, small enough to be overcome. They live in damp obscure places, under logs of wood or stones, and are nocturnal, shunning, like the _Geophilidae_, exposure to light; and as in the _Geophilidae_, the females guard their eggs and young until the latter disperse to lead an independent life.

_Order 3. Craterostigmomorpha_.--Chilopods with twenty-one tergal plates as in the typical genera of Scolopendromorpha, but with only fifteen pairs of legs as in the Lithobiomorpha. As in some members of the latter order there is a single ocellus on each side of the head, the penultimate and antepenultimate segments of the toxicognaths are complete on the postaxial side of the appendage, and spiracles are present upon the 3rd, 5th, 8th, 10th, 12th and 14th leg-bearing somites. In the size and shape of the head, of the toxicognaths, of the tergal plate of this somite, and of the first leg-bearing somite, great similarity to some genera of Geophilomorpha (e.g. _Mecistocephalus_) is presented; but in the structure of the posterior end of the body this order differs from all the other orders of Chilopoda. The skeletal elements of the last leg-bearing segment are welded together to form a subcylindrical tube, and the genital and anal somites are represented by a pair of chitinous valves capable of opening below for the escape of the genital and intestinal products.

[Illustration: After Pocock. Q.J.M.S. vol. 45, pl. 23, 1902.

FIG. 8.

A, Anterior end of _Craterostigmus_ from above.

a, Basal segments of antennae.

c, Cephalic plate with eyes (o).

t.tox, Tergal plate of somite bearing toxicognaths (tox).

t.lg.1, Tergal plate of somite bearing legs of the first pair.

B, Maxillae.

C, Palpognath.

D, Toxicognath.

E, Last segment with genital capsule (g.c), and basal segments of legs of 14th and 15th pairs (lg. 14, lg. 15).]

This order, containing the family _Craterostigmidae_, is based upon a remarkable genus and species _Craterostigmus tasmanianus_, of which only two specimens are known. These were collected under stones upon the summit of Mount Rumney in Tasmania. They are about 1½ in. in length; but nothing has been recorded of their habits. The chief morphological interest attaching to _Craterostigmus_ is that, apart from certain structural peculiarities of its own, it presents features previously believed to be found exclusively either in the Scolopendromorpha, or the Geophilomorpha, or the Lithobiomorpha; and it shows how the Lithobiomorpha may be derived from a Scolopendromorphous type most nearly resembling _Plutonium_ by the excalation of the third, sixth, ninth, eleventh, fourteenth and seventeenth leg-bearing somites.

_Order 4. Lithobiomorpha._ Chilopoda with fifteen pairs of leg-bearing somites differentiated into larger and smaller, the 1st, 3rd, 5th, 7th, 8th, 10th, 12th and 14th being large, the others small. Spiracles present upon all the larger with the exception sometimes of the 1st. The toxicognaths are relatively weaker than in the orders hitherto considered, and have their basal segments less firmly fused mesially. In correlation with their weaker muscularity the first leg-bearing segment is relatively small. The gonopods, present and usually jointed in both sexes, are especially well developed and forcipate in the female, and arise from a large ventral plate resulting from the fusion of their coxae with the sternum of the genital somite. The antennae are many-jointed, and there is a single ocellus or a cluster of ocelli on each side of the head. The coxae of the legs are large, and those of the last four or five pairs usually contain glands opening by large orifices. The newly-hatched young has only seven pairs of legs, the remaining pairs being successively added as growth proceeds.

The genera of this order are divisible into three families, the _Lithobiidae_ (_Lithobius, Bothropolys_), _Henicopidae_ (_Henicops, Haasiella_), the _Cermatobiidae_ (_Cermatobius_). _Cermatobius_, based upon a single species, _martensii_, from the isl. of Adenara, is of peculiar interest, since in the absence of coxal pores, and the length and multi-articulation of the antennae and tarsal segments, it approaches more nearly to _Scutigera_ than does any other pleurostigmous Chilopod. It is also stated that the spiracles have assumed a more dorsal position, thus foreshadowing the completely dorsal situation they have taken up in the Notostigma. The _Henicopidae_, containing centipedes of small size, attains its maximum of development in the southern continents and islands, more particularly Australia, New Zealand, South Africa and South America. One genus (_Lamyctes_) however, occurs in Europe. The _Lithobiidae_, on the contrary, are almost exclusively northern in range, being particularly abundant and of large size individually in Europe, extra-tropical Asia, and North and Central America. In habits the _Lithobiidae_ closely resemble the _Scolopendridae_. They are, however, comparatively far more agile with their shorter, more compact bodies and stronger legs. They are mostly of small size, the largest species, _Lithobius fusciatus_, of south Europe measuring only 2 in. in length of body. The females do not guard their eggs, but coat them with soil and leave them to their fate.

SUBCLASS 2, NOTOSTIGMA.--Chilopods with a series of median dorsal tracheal sacs furnished with tubes dipping into the pericardial blood space, and opening each by an unpaired spiracle upon the 1st, 3rd, 5th, 8th, 10th, 12th and 14th leg-bearing somites. This characteristic is accompanied by the complete disappearance of the tergum of the 7th, either by fusion with that of the 8th or by excalation, and by the evanescence of the terga of the 2nd, 4th, 6th, 9th, 11th and 13th pedigerous somites. The preantennal area of the head is not strongly reflexed inferiorly, and the eyes are large and compound. The maxillae are long and have a sensory organ, the palpognaths are long, spiny and composed of five segments, like the primitive Chilopod leg, and the toxicognaths have their basal segments disunited and independently movable. Gonopods duplicated in the male.