Part 4
The next family, _Veneridæ_, have long respiratory siphons and a sinuated pallial line. Many of this tribe are very beautiful in form and colouring, and most of them have very hard strong shells. The valves are united above by an external ligament, and the hinge-plate is toothed. Nearly all of them live buried an inch or two beneath the sand or mud, but a few are found burrowing in rocks. Probably the majority of the species of this family might be used as food. _Venus verrucosa_, of our own southern shores, is frequently eaten both in this country and abroad; and _Venus mercenaria_ (Case 189) is commonly sold in the markets of Philadelphia and New York. _Cytherea lusoria_ (Case 185) also forms a favourite article of diet among the poorer classes in Japan, and several kinds are eaten by the natives of New Zealand and other countries.
[Sidenote: Cases 192–194.]
Some of the “Cockles” (_Cardiidæ_) from warm latitudes are highly coloured and adorned with most beautiful sculpture. Probably the majority are eatable, as the common cockle (_Cardium edule_) of the British coast. The foot of these molluscs is very large, bent, and used for leaping. The siphons are short and fringed at the margins.
[Sidenote: Cases 195–196.]
The _Tridacnidæ_, or true Clams, differ from other Bivalves with united mantle-margins in having but a central adductor muscle. In the typical species the animal is attached to the rocks by a “byssus,” a strong fibrous structure which passes through an aperture at the upper part of the shell. A species found in the Red Sea, _T. elongata_, is eaten by the natives, and the shell employed for the manufacture of lime. _Tridacna gigas_, the largest known bivalved mollusc, sometimes weighs over 500 lb., that exhibited in the upright cases at the entrance of the Gallery being 310 lb. in weight. A large pair bordered with gilt copper are used as _bénitiers_ or holy-water vessels in the church of St. Sulpice in Paris. _Tridacnæ_ are found associated in large numbers in lagoons, among coral-reefs in the Eastern and Pacific Seas. The animals are described as presenting a beautiful iridescent glare of blue, violet, and yellow variegated with fantastic markings.
[Illustration:
Fig. 33.
Left valve of the Giant Clam (_Tridacna gigas_).
Length, 36 inches. Weight, 154 lb.; weight of the two valves, 310 lb. ]
[Sidenote: Case 196.]
The genus _Chama_ consists of tropical species, which are found fixed to corals, rocks, etc. Nevertheless, they have a small bent foot, but what purpose it serves is difficult to conceive.
In this place attention should be called to the _Hippuritidæ_ and _Radiolitidæ_, very remarkable extinct families of bivalved molluscs which occur abundantly in the cretaceous strata of southern and eastern Europe, Egypt, etc. They are remarkable for the solidity of the shells, the relatively small space occupied by the animal, and the complicated character of the hinge and processes bearing the adductor muscles. They are usually classed near to the _Chamidæ_, but their true position as regards living Mollusca is very problematical. A fine series of these shells is exhibited in Gallery VIII., wall-case 5, in the Geological Department.
[Sidenote: Cases 198–199.]
The _Myidæ_, popularly known as “_Gapers_,” on account of their valves being open at one or both ends, have the mantle united all round, except where the small foot is protruded. The siphons are very long, united almost to the ends, and covered with a coarse wrinkled outer skin. They bury themselves in mud and sand at low-water mark or in shallow water. The species are few in number, and chiefly from the shores of northern countries. _Mya arenaria_ of our own coasts is largely eaten in some parts of Europe and North America.
[Illustration:
Fig. 34.
British Gaper (_Mya truncata_).
_a_, foot; _b_, siphon-sheath; _c_, exhalant siphon; _d_, inhalant siphon; _e_, umbones or beaks; _f_, anterior, _g_, posterior end of shell. ]
[Sidenote: Case 198.]
The _Corbulæ_ (Case 198) have one valve larger than the other and are like little _Myæ_, but the valves _are almost closed_ and their siphons are very short.
[Illustration:
Fig. 35.
British Razor-shell (_Solen siliqua_).
_a_, foot; _b_, mantle; _c_, inhalant siphon; _d_, exhalant siphon; _e_, shell. ]
[Sidenote: Cases 199–201.]
Many of the _Solenidæ_, or Razor-shells, possess very elongated shells, and are remarkable for the great development of the foot, which can be pointed or contracted as may be required for boring into sand. By means of this powerful foot the animals, when disturbed, bore with such rapidity and to such a depth that their capture is a matter of great difficulty; and even when seized they hold on so tightly that at times they suffer their foot to be torn off rather than be captured. They not only burrow in sand, but also have the power of darting through the water, like the Scallops. Solens were considered a dainty dish by the ancient Greeks, and numbers are still eaten by the poorer coast-population of this country and abroad.
[Sidenote: Cases 201–202.]
The _Pholadidæ_, or Piddocks, are very remarkable shells, of an unusually complicated structure, some having the power of boring into rocks, wood, mud, sand, etc. Their shells are white, adorned with prickly sculpture, and, although thin, are strong. The foot is believed to be the principal excavating instrument, but the shell no doubt is used as a file to enlarge the hole as the creature grows. These animals are brightly phosphorescent; and certain species are eaten at many places on the shores of the Mediterranean. They appear to be indifferent as regards the material they bore into; for the common _Pholas dactylus_ (Fig. 36) of our own shores has been found in slate-rocks, mica-schist, coal-shale, new red sandstone, chalk, marl, peat, and submarine wood. The siphons are long in the Piddocks, united except near the end, and enclosed in tough skin. The species are world-wide in their distribution, and several are found fossil in some of the Tertiary formations.
[Illustration:
Fig. 36.
Piddock, or Borer (_Pholas dactylus_). (From the British coast.)
1. Animal in the shell: _a_, foot; _b_, siphons; _c_, inhalant orifice; _d_, exhalant orifice.
2. Shell: _e_, accessory valves or plates. ]
[Sidenote: Case 202.]
The _Teredinidæ_, or Ship-worms, are also borers, like the Pholads, but do not perforate rocks. They are principally wood-borers; the large _Kuphus arenaria_, which is an exception, living buried in the sand. The ship-worm has a long worm-like body, from 6 to 12 inches in length, which is more or less enclosed in a thin shelly tube or sheath. The true bivalved shell is at the thicker end, and protects the mouth, labial palps, the liver, and other internal organs. At the opposite, or more slender, end of the animal, the mantle is produced into two small tubes, one of which conveys the water to the gills, whilst through the other the water is expelled, charged with the woody pulp excavated by the foot. At the end there is a pair of pallets, or paddles as they are sometimes termed, which are probably used as a means of defence, in closing the shelly tube after the contraction of the siphons.
[Illustration:
Fig. 37.
Ship-worm (_Teredo norvegica_). Case 202.
_a_, animal, removed from its shelly tube: _p_, _p_, pallets; _s_, exhalant siphon; _s′_, inhalant siphon.
_b_, _c_, different aspects of the shell. ]
These animals are most destructive to ships, piers, etc.; and wood, which is not protected by metal, when once attacked, is soon riddled through and through. They work either with or across the grain, and although the holes may be all but touching, they seldom appear to run into one another.
[Illustration:
Fig. 38.
Watering-pot Shell (_Brechites vaginifer_).
Case 204.
_a_, bivalve shell of the very young animal. ]
The “Watering-pot shell” (_Brechites_), of the family _Clavagellidæ_, is a very remarkable structure, and unlike the shell of an ordinary bivalved mollusc. On looking carefully, however, near the perforated end (the rose), two small valves will be seen imbedded in the surface. They are found with the rose downwards buried in mud or sand at low water on the shores of the Indian and Pacific Oceans.
SEPTIBRANCHIA. (Fig. 25, D.)
[Sidenote: Case 204.]
The members of this order differ from other Lamellibranchs in having the gill-plates represented by a muscular _septum_. They are provided with two respiratory siphons and two adductor muscles, and the edges of the mantle-lobes are connected at three points. The families _Poromyidæ_, and _Cuspidariidæ_, constitute this order. The species are all small, without colour-markings, are world-wide in their distribution and occur at all depths.
Class V.—CEPHALOPODA.[9]
[Illustration:
Fig. 39.
A, the upper, B, the lower beak of _Architeuthis monachus_; one-third natural size. ]
[Sidenote: Cases 205–208.]
This Class includes the Octopus or Polypus, Cuttlefish, Squid, Spirula, the Paper and Pearly Nautilus. The body of the animal consists of a muscular sac, in the cavity of which the viscera are placed. In front of the body projects the head, which, in species belonging to the two-gilled section of the Class, is surrounded by eight or ten fleshy arms. A wide aperture below the head admits the water to the gills or branchiæ, which are situated in the interior of the sac, whilst a short tube, the so-called funnel or siphuncle, projects from the opening of the mantle—the water and various excretions being expelled through this tube, especially also an ink-like fluid, which is discharged by all Cephalopods (except _Nautilus_) when disturbed, in order to darken the water and thus escape their enemies. The centre of the head, between the base of the arms, is occupied by the mouth, which is armed with two horny or calcareous jaws, similar to the beak of a parrot (Fig. 39). The two large eyes are placed on the sides of the head. The arms or feet are more or less elongate, capable of movement in any direction, and, except in _Nautilus_, furnished on one side with numerous suckers, by means of which the animal attaches itself most securely to anything it may seize; they are employed in capturing food and in walking. Cephalopods walk in any direction head downwards, but can swim backwards only, being propelled in that direction by the water which they discharge with force through the funnel out of their branchial cavity. They are divided, according to the number of their gills (which is either two or four), into _Dibranchia_ and _Tetrabranchia_. Of the latter but one representative now exists, viz., the Pearly Nautilus, all other living Cephalopods being provided with but two gills, placed one on each side of the body within the mantle, as may be seen in the wax model of _Sepia officinalis_ (Case 207). The two-gilled section comprises forms with eight arms, as _Argonauta_ and _Octopus_, and others with ten arms, viz., the Cuttlefishes (_Sepia_) (Fig. 43), the Squids (_Loligo_, _Ommatostrephes_, _Sepiola_, _Chiroteuthis_, etc.), and _Spirula_. The “shell” of the Paper-Nautilus, or _Argonauta_, is too well known to require any description. Unlike the shells of other Mollusca, it is not attached to the animal by a special muscle, but is held on to the body by two of the arms, which are dilated and specially adapted for this purpose. Only the female Argonaut is provided with a shell, the male being shellless and a much smaller creature. The Argonaut-shell is therefore not a true shell, but simply a receptacle for the ova, serving at the same time for the protection of the parent.
[Illustration:
Fig 40.
The Common Octopus (_Polypus vulgaris_), resting. ]
[Illustration:
Fig. 41.
_Sepiola scandica_ (Natural size). British. ]
_Chiroteuthis Veranyi_ is remarkable on account of the great length of the tentacular arms. These are non-retractile and are employed to seize their prey when at a distance.
[Illustration:
Fig. 42.
_Chiroteuthis Veranyi_ (much reduced).
_a_, general view of animal; _b_, magnified view of pedunculated sucker of the terminal club of the tentacular arms; _c_, internal shell or gladius. ]
The species of Octopus are found on the shores of almost all temperate and tropical seas; they do not attain to a large size, and are without the internal shell or “bone” which is found in the mantle of many Cephalopods. That of the Cuttlefish or _Sepia_ (Fig. 43 _a_) is found in abundance on our coasts; it is composed of numberless layers of a friable calcareous substance. That of the Squid tribe is of quite another character, consisting of an elongate thin horny plate, and strengthened by one or more thickened ribs, in some species somewhat resembling a quill-pen. Some species of this pen-bearing class related to the Common Squid attain an immense size. One was captured off the Irish coast in June, 1875 (probably _Architeuthis harveyi_), with the shorter arms 8 feet in length and 15 inches in circumference at the base, the two tentacular arms having a total length of 30 feet. The powerful beak measured about 4 inches across. Thus from the tip of the tail to the end of the tentacular arms this wonderful monster must have measured something like 40 feet in length. Other very large specimens of _Architeuthis_ have been captured on the coasts of Newfoundland and Labrador. Two specimens stranded on the south coast of Newfoundland, in the winter of 1870–1871, measured respectively 40 and 47 feet. Another, cast ashore at Bonavista Bay in December, 1873, had a very stout body 14 feet long, arms 10 feet, and tentacles 24 feet in length. These are only a few of the many instances of the capture of gigantic Cephalopods, which occur not only in the North-Atlantic Ocean, but also in tropical seas. Their appearance in mid-ocean may, in some instances, have given rise to the tales of “Sea-serpents.” Specimens much smaller than those mentioned above have attacked men, and pearl-fishers are in constant fear of them. One of the arms of a large Squid (_Architeuthis harveyi?_), which is supposed to have been found off the coast of South America, is exhibited in the black upright Case A at the side of the room.
[Illustration:
Fig. 43.
The Common Cuttlefish (_Sepia officinalis_), and its shell or bone (_a_). ]
The shells of _Spirula_ (Fig. 44) have been long known, and are scattered in thousands on the shores of New Zealand and other islands in the Pacific Ocean, and they are also found in the Indian and Atlantic Oceans, occasionally drifting on the coast of Devon and Cornwall. Notwithstanding the abundance of the shells, very few specimens of the perfect animal have been captured. The loosely-coiled shell resembles a ram’s horn, and is divided into a number of segments by fine concave partitions, like the shell of _Nautilus_, each one pierced by a slender tube or siphon. It is placed at the hinder end of the body, and is covered with so thin a skin, that a small portion of it appears to be exposed both in front and behind. Absolutely nothing is known of the habits of this very interesting creature, although probably they are somewhat similar to those of other Cephalopods.
[Illustration:
Fig. 44.
The Spirula (_Spirula peronii_). (From the Indian and Pacific Oceans.)
1. Animal: _a_, portions of the shell exposed in front and behind; _b_, the funnel or siphuncle. 2. Side view of shell. 3. Shell in section, to show partitions or septa. ]
The _Nautilus_ (Fig. 45), of which several shells (Case 208) and a perfect animal in spirit (black upright case A) are exhibited, is an inhabitant of the Indo-Pacific Ocean, and differs from all other living Cephalopods in being provided with four instead of two gills, and, instead of eight or ten arms with suckers and hooks, has a number of small retractile feelers. The Nautilus occasionally swims, like other members of its class, at the surface of the sea, but mostly crawls about leisurely on its feet at the bottom in search of food, which consists chiefly of small crabs or Mollusca, which it crushes with its strong calcareous parrot-like mandibles.
[Illustration:
Fig. 45.
The Pearly Nautilus (_Nautilus pompilius_).
_a_, body; _b_, siphuncle; _c_, eye; _d_, hood; _e_, tentacles; _f_, muscle of attachment to the shell; _g_, siphon. ]
The chambered shell is pearly within, and covered with an external calcareous layer. The chambers are connected by a slender tube or siphon, the function of which is not at present thoroughly understood. The septa, or partitions across the shell, indicate periods of growth. When the Nautilus outgrows the capacity of the outer chamber, in which it resides, it constructs a new one of larger size, separating the additional chamber from the preceding one by a transverse partition.
A series of Cephalopods preserved in spirit is exhibited in the black upright case at the side of the room.
ALPHABETICAL INDEX OF THE FAMILIES AND PRINCIPAL GENERA OF MOLLUSCA EXHIBITED IN THE SHELL GALLERY.
This Index has been compiled to assist the numerous visitors, who wish to examine and determine specimens of shells, in finding, without trouble or loss of time, the Cases in which the genera are placed. Subgeneric terms are omitted, as they do not fall within the scope of this “Guide.”
Acanthina, 75
Achatina, 130–131
Achatinella, 134, 135
Acmæa, 4
Actæon, 94
Ætheria, 178
Amphibola, 98
Amphiperas, 35
Ampullaria, 25, 26
Amussium, 159
Anatina, 203
Ancylus, 99
Anodonta, 166, 167
Anomia, 137
Anostoma, 128
Aplacophora, 3
Aplustrum, 96
Aplysia = Tethys, 96
Aporrhais, 49
Arca, 139
Arctica, 162
Argonauta, 205, 206
Arion, 107
Aspergillum (= Brechites), 204
Astarte, 161
Atlanta, 94
Auriculidæ, 97, 98
Avicula = Pteria, 145
Batissa, 165
Brechites, 204
Bryopa, 204
Buccinum, 65
Buliminus, 128
Bulimulus, 122
Bulimus = Strophocheilus, 120
Bullidæ, 95
Calyptræidæ, 31, 32
Cancellaria, 77
Capulus, 31
Cardita, 160
Cardium, 192–194
Carinaria, 94
Cassis, 55–56
Cerion, 129
Cerithiidæ, 42–44
Chætoderma, 3
Chama, 196
Chitonidæ, 1–3
Chrysodomus, 64
Circe, 187
Clausilia, 129–130
Clavagella = Bryopa, 204
Columbella, 69
Conus, 89–94
Coralliophila, 76
Corbicula, 164
Corbis, 163
Corbula, 198
Crassatella, 161
Crenatula, 147
Crenella, 145
Cucullæa, 140
Cuma, 75
Cuspidaria, 204
Cyclophoridæ, 23, 25
Cyclostomatidæ = Pomatiidæ, 28, 29
Cylindrella, 127
Cypræa, 32–35
Cyprina = Arctica, 162
Cyrena, 164
Cythera = Meretrix, 185
Delphinula, 12
Dentalium, 136
Despoena, 22
Diplodonta, 163
Dolium, 56, 57
Donax, 182
Dosinia, 188
Dreissensia, 179
Eburna, 67
Emarginula, 7
Ennea, 103
Eucalodium, 129
Eulima, 46
Fasciolaria, 59
Ficula = Pirula, 57
Fissurella, 8
Fulgur, 61
Fusus, 57, 58
Gadinia, 99
Galatea, 165
Galeomma, 163
Gastrochæna, 201
Gena, 11
Glandina = Oleacina, 102
Glauconome, 192
Glycymeris, 141
Haliotis, 9–11
Haminea, 95
Harpa, 84
Helicarion, 104
Helicidæ, 107–121
Helicina, 21
Hemifusus, 61
Heteropoda, 24
Hinnites, 159
Hydatina, 96
Ianthina, 38
Isocardia, 162
Isognomon = Melina, 147
Kellia, 163
Latiaxis, 74
Latirus, 59
Leda = Nuculana, 137
Lepeta, 4
Lepton, 163
Lima, 156
Limax, 106
Limnæidæ, 99–102
Limopsis, 141
Lithodomus, 144
Littorina, 27
Loligo, 207
Lotorium, 53, 54
Lucinidæ, 162, 163
Lutraria, 199
Lyonsia, 203
Mactridæ, 183–185
Magilus, 77
Malletia, 137
Malleus, 146
Marginella, 83, 84
Melaniidæ, 38–41
Meleagrina, 146
Melina, 147
Melongena, 61
Meretrix, 185
Mesodesma, 182
Mitridæ, 61–64
Modiola, 143, 144
Modiolarca, 145
Modiolaria, 145
Monoceros = Acanthina, 75
Montacuta, 163
Murex, 70–73
Mutela, 177
Mya, 198
Myadora, 203
Mycetopus, 176
Myochama, 203
Mytilus, 142, 143
Nassa, 67, 68
Naticidæ, 36, 37
Nautilus, 208
Navicella = Septaria, 20, 21
Neæra = Cuspidaria, 204
Neomenia, 3
Nerita, 18, 19
Neritina, 19–20
Nucleobranchiata = Heteropoda, 84
Nucula, 137
Nuculana, 137
Nudibranchiata, 97
Octopus, 205
Oleacina, 102
Olividæ, 81–83
Ostrea, 152–154
Ovulum = Amphiperas, 35
Paludina = Vivipara, 22
Paludomus, 41
Pandora, 202
Panopea, 201
Partula, 126, 127
Patella, 4–7
Pectinidæ, 156–160
Pectunculus = Glycymeris, 141
Pedum, 156
Periploma, 203
Petricola, 192
Philine, 96
Pholadidæ, 201, 202
Pholadomya, 204
Pholas, 201, 202
Phorus (= Xenophora), 49
Physa, 101
Pinna, 148–152
Pirula, 57
Placuna, 138
Planaxis, 44
Planorbis, 100
Pleurotomaria, 9
Pleurotomidæ, 85–87
Plicatula, 154
Pomatiidæ, 28, 29
Proserpina = Despoena, 22
Psammobia, 197
Pteria, 145
Pterocera, 51, 52
Pteropoda, 96
Puncturella, 7
Pupa, 128
Purpura, 74, 75
Pyramidellidæ, 45
Ranella, 54
Ricinula = Sistrum, 76
Ringicula, 95
Rissoiidæ, 30
Rocellaria, 201
Rostellaria, 52
Rotella, 15, 16
Saxicava, 201
Scala, 44, 45
Scalaria = Scala, 44, 45
Scaphander, 95
Scaphopoda, 136
Scintilla, 163
Scutum, 7
Semele, 181
Sepia, 206, 207
Septaria, 20, 21
Septifer, 143
Siliquaria, 47
Siphonaria, 98, 99
Sistrum, 76
Solarium, 45
Solenidæ, 199–201
Solenomya, 137
Sphærium, 165
Spirula, 207
Spondylus, 154–156 Stenogyra, 133
Stilifer, 46
Stomatella, 11
Streptaxis, 102
Strombus, 49–51
Strophia = Cerion, 129
Strophocheilus, 120
Struthiolaria, 49
Succinea, 135
Sunetta, 187
Sycotypus, 61
Tapes, 190,191
Tellinidæ, 179–181
Terebellum, 52
Terebridæ, 87–89
Teredo, 202
Testacella, 102
Tethys, 96
Thracia, 203
Trichotropis, 37
Tridacna, 195
Trigonia, 141
Triton = Lotorium, 53, 54
Trochidæ, 12–15
Trophon, 70
Truncatella, 30
Tugonia, 198
Turbinellidæ, 60,61
Turbinidæ, 16–18
Turritella, 48
Typhis, 70
Umbraculum, 97
Umbrella = Umbraculum, 97
Ungulina, 163
Unionidæ, 166–178
Valvata, 30
Vanicoro, 49
Velutina, 37
Veneridæ, 185–191
Venerupis, 192
Venus, 188
Vermetidæ, 46, 47
Vitrina, 106
Vivipara, 22
Volutidæ, 78–81
Vulsella, 147
Xenophora, 49
Yoldia, 137
POLYZOA.
(*) An asterisk against names of species denotes that specimens of these species are in the upright part of Case A and preserved in spirit.
[Sidenote: Upright Table-Cases A and B, at south end of Shell Gallery.]
From a casual glance at the contents of these cases, it might be supposed that many of the specimens exhibited therein were seaweeds; but a closer inspection, especially with a lens, will reveal structure of a kind not to be found in any plant.
Let us select for examination _Flustra foliacea_, the Broad-leaved Hornwrack or Sea-Mat (Fig. 1), (Case A 1), commonly to be found among heaps of seaweed cast up on sandy shores round our coasts.
[Illustration:
Fig. 1.
_Flustra foliacea._ A, natural size; B′, portion magnified in B; B, magnified 30 diameters.
_a_, avicularium; _o_, ovicell.
[‘The Cambridge Natural History.’] ]
The brown horny fronds, which vary in width, branch upwards from a narrow flat stem attached at its base to stones and shells. Both surfaces of the fronds show a fine network pattern formed by the edges of little oblong boxes or cells termed zoœcia,[10] arranged in longitudinal parallel rows and forming a double layer back to back. The cells are broad and rounded above, narrow and truncate below, and each is roofed in by a transparent membrane with a semicircular lid or operculum situated near the upper end; four short stout spines spring from the margin in this neighbourhood. When the surface of a living frond is examined in sea-water, here and there a bundle of tentacles may be observed pushing up a lid, slowly emerging and expanding into a bell-shaped coronet; on the least alarm the tentacles are rapidly withdrawn into the cell and the lid shut. The flexible protrusible region of the cell is termed the tentacle sheath. The relation of the cell to the tentacle sheath (Figs. 2, 3) may be roughly compared to a glove finger, stiff below, but flexible at the end, and surmounted by a crown of bristles; on pulling down the glove-finger tip, the tentacles will also be drawn in, and will lie in a sheath formed by the invaginated portion of glove finger. The lid which closes over the tentacle sheath is only found in the Sub-order Chilostomata to which _Flustra_ belongs. The area of the tentacle sheath whence the tentacles arise is termed the lophophore.[11]
[Illustration:
Figs. 2, 3, diagrams representing polypide in cell. Fig. 2, tentacle-sheath protruded. Fig. 3, ditto, retracted; _a_, tentacles; _b_, tentacle-sheath; _c_, mouth; _d_, gullet; _e_, stomach; _f_, vent; _g_, retractor muscle; _h_, funiculus; _l_, ovary; _k_, testis; _l_, lid or operculum; nerve ganglion is between mouth and vent. Fig. 4, polypide extracted from cell; _d_, pharynx; _e_, stomach; _f_, anus (after Van Beneden). Fig. 5, section (partly diagrammatic) of frond of _Flustra_, showing cells back to back. ]