CHAPTER IX

MOLLUSKS

THE FRESH-WATER MUSSEL

SUGGESTIONS.--The mussel is usually easy to procure from streams and lakes by raking or dredging. In cities the hard-shelled clam, or quahog, is for sale at the markets, and the following descriptions apply to the anodon, unio, or quahog, with slight changes in regard to the siphons. Mussels can be kept alive for a long time in a tub with sand in the bottom. Pairs of shells should be at hand for study.

=External Features.=--The shell is an elongated oval, broader and blunter at one end (Fig. 188). Why does the animal close its shell? Does it open the shell? Why? Does it thrust the foot forward and pull up to it, or thrust the foot back and push? (Mussels and clams have no bones.) Does it go with the blunt or the more tapering end of the shell forward? (Fig. 188.) Can a mussel swim? Why, or why not?

Lay the shells, fitted together, in your hand with _the hinge side away from you and the blunt end to the left_ (Fig. 188). Is the right or the left shell uppermost? Which is the top, or dorsal, side? Which is the front, or anterior, end? Is the straight edge at the top or the bottom? Our word “valve” is derived from a word meaning shell, because the Romans used shells for valves in pumps. Is the mussel a univalve or a bivalve? Which kind is the oyster? The snail?

Does the mussel have _bilateral symmetry_? Can you find a _horny covering_, or epidermis, over the limy shell of a fresh specimen? Why is it necessary? Does water dissolve lime? Horn? Find a bare spot. Does any of the shell appear to be missing there?

The bare projection on each shell is called the _umbo_. Is the umbo near the ventral or the dorsal line? The posterior or anterior end? Is the surface of the umbones worn? Do the umbones rub against the sand as the mussel plows its way along? How are the shells held together? Where is the _ligament_ attached? (Fig. 189.) Is it opposite the umbones or more to the front or rear? (Fig. 189.) Is the ligament of the same material as the shell? Is the ligament in a compressed condition when the shell is open or when it is closed? (Fig. 189.) When is the muscle relaxed?

[Illustration: FIG. 188.--ANODON, or fresh-water mussel.]

[Illustration: FIG. 189.--DIAGRAM OF SHELL open and closed, showing muscle, _m_, and ligament, _b_.]

[Illustration: FIG. 190.--MUSSEL crawling in sand.]

Notice the _lines_ on the outside of the shell (Figs. 188 and 190). What point do they surround? They are _lines of growth_. Was each line once the margin of the shell? If the shell should increase in size, what would the present margin become? (Fig. 191.) Does growth take place on the margin only? Did the shell grow thicker as it grew larger? Where is it thinnest?

=Draw= the outside of the shell from the side. Draw a dorsal view. By the drawings write the names of the margins of the shell (p. 98) and of other parts learned, using lines to indicate the location of the parts.

Study the surface of the shell inside and out. The inside is called _mother-of-pearl_. Is it of lime? Is the deeper layer of the shell of lime? (When weak hydrochloric acid or strong vinegar is dropped on limy substances, a gas, carbon dioxid, bubbles up.) Compare the thickness of the _epidermal layer_, the middle _chalky layer_, and the inner, _pearly layer_.

[Illustration: FIG. 191.--DIAGRAM. Change of points of attachment of muscles as mussel enlarges. (Morgan.)]

=Anatomy of the Mussel.=--What parts protrude at any time beyond the edge of the shell? (Fig. 190.) The shell is secreted by two folds of the outer layer of the soft body of the mussel. These large, flaplike folds hang down on each side, and are called the _mantle_. The two great flaps of the mantle hang down lower than the rest of the body and line the shell which it secretes (Fig. 192). The epidermis of the mantle secretes the shell just as the epidermis of the crawfish secretes its crust. Can you find the pallial line, or the line to which the mantle extended on each shell when the animal was alive? A free portion of the mantle extended like a fringe below the pallial line.

[Illustration: FIG. 192.--CROSS SECTION OF MUSSEL. (Diagram, after Parker.)]

The shells were held together by two large _adductor muscles_. The anterior adductor (Fig. 193) is near the front end, above the foot. The posterior adductor is toward the rear end, but not so near the end as the anterior. Can you find both _muscle scars_ in the shells? Are they nearer the ventral or dorsal surface? The points of attachment traveled downward and farther apart as the animal grew (see Fig. 191). Higher than the larger scars are small scars, or impressions, where the protractor and retractor muscles that extend and draw in the foot were attached.

[Illustration: FIG. 193.--ANATOMY OF MUSSEL. (Beddard.)]

The muscular _foot_ extends downward in the middle, halfway between the shells (Fig. 193). On each side of the foot and behind it hang down the two pairs of =gills=, the outer pair and the inner pair (Fig. 192). They may be compared to four V-shaped troughs with their sides full of holes. The water enters the troughs through the holes and overflows above. Is there a marked difference in the size of the two pairs of gills? A kind of chamber for the gills is made by the joining of the mantle flaps below, along the ventral line. The mantle edges are separated at two places, leaving openings called _exhalent_ and _inhalent siphons_.

Fresh water with its oxygen, propelled by _cilia_ at the opening and on the gills, enters through the lower or inhalent siphon, passes between the gills, and goes to an upper passage, leaving the gill chamber by a slit which separates the gills from the foot. For this passage, see arrow (Fig. 194). The movement of the water is opposite to the way the arrow points. After going upward and backward, the water emerges by the exhalent siphon. The gills originally consisted of a great number of filaments. These are now united, but not completely so, and the gills still have a perforated or lattice structure. Thus they present a large surface for absorbing oxygen from the water.

[Illustration: FIG. 194.--MUSSEL.

_A_, left shell and mantle flap removed.

_B_, section through body.

=Question:= Guided by other figures, identify the parts to which lines are drawn.]

The =mouth= is in front of the foot, between it and the anterior adductor muscle (Fig. 194). On each side of the mouth are the _labial palps_, which are lateral lips (Fig. 195). They have cilia which convey the food to the mouth after the inhalent siphon has sent food beyond the gill chamber and near to the mouth. Thus both food and oxygen enter at the inhalent siphon. The foot is in the position of a lower lip, and if regarded as a greatly extended lower lip, the animal may be said to have what is to us the absurd habit of using its lower lip as a foot. The foot is sometimes said to be hatchet-shaped (Fig. 195). Do you see any resemblance? Does the foot penetrate deep or shallow into the sand? (Fig. 190.) Why, or why not?

[Illustration: FIG. 195.--MUSSEL. From below. Level cut across both shells.

_Se_, palp; _P_, foot; _O_, mouth; _G_, liver; _Gg_, _Vg_, _Pg_, ganglia.]

[Illustration: FIG. 196.--HEART OF MUSSEL, with intestine passing through it.]

[Illustration: FIG. 197.]

The =food tube= of the mussel is comparatively simple. Behind the mouth it enlarges into a swelling called the _stomach_ (Fig. 193). The bile ducts of the neighboring liver empty into the stomach. The _intestine_ makes several turns in the substance of the upper part of the foot, and then passing upward, it runs approximately straight to the vent (or anus), which is in the wall of the exhalent siphon. The intestine not only runs through the _pericardial cavity_ (celome) surrounding the heart, but through the ventricle of the heart itself (Fig. 196).

The =kidneys= consist of tubes which open into the pericardial chamber above and into the gill chamber below (_Neph._, Fig. 193). The tubes are surrounded by numerous blood vessels (Fig. 198) and carry off the waste matter from the blood.

The =nervous system= consists of _three pairs of ganglia_ and nerves (Fig. 197). The ganglia are distinguishable because of their orange color. The pedal ganglia on the front of the foot are easily seen also; the visceral ganglia on the posterior adductor muscle may be seen without removing the mussel from the shell (Fig. 193). The reproductive organs open into the rear portion of the gill cavity (Fig. 193). The sperms, having been set free in the water, are drawn into the ova by the same current that brings the food. The eggs are hatched in the gills. After a while the young mussels go out through the siphon.

=Summary.=--In the gills (Fig. 198) the blood gains what? Loses what? From the digestive tube the blood absorbs nourishment. In the kidneys the blood is partly purified by the loss of nitrogenous waste.

[Illustration: FIG. 198.--DIAGRAM OF MUSSEL CUT ACROSS, showing mantle, _ma_; gills, _kie_; foot, _f_; heart, _h_; intestine, _ed_.]

The cilia of the fringes on the inhalent, or lower, siphon, vibrate continually and drive water and food particles into the mouth cavity. Food particles that are brought near the labial palps are conveyed by them to the mouth. As the water passes along the perforated gills, its oxygen is absorbed; the mantle also absorbs oxygen from the water as it passes. The water, as stated before, goes next through a passage between the foot and palp into the cavity above the gills and on out through the exhalent siphon. By stirring the water, or placing a drop of ink near the siphons of a mussel kept in a tub, the direction of its flow may be seen. The pulsations of the heart are plainly visible in a living mollusk.

=Habits of the Mussel.=--Is it abundant in clear or muddy water; swift, still, or slightly moving water? Describe its track or furrow. What is its rate of travel? Can you distinguish the spots where the foot was attached to the ground? How long is one “step” compared to the length of the shell? The animal usually has the valves opened that it may breathe and eat. The hinge ligament acts like the case spring of a watch, and holds the valves open unless the adductor muscles draw them together (Fig. 189).

When the mussel first hatches from the egg, it has a triangular shell. It soon attaches itself to some fish and thus travels about; after two months it drops to the bottom again.

=Other Mollusca.=--The _oyster’s_ shells are not an exact pair, the shell which lies upon the bottom being hollowed out to contain the body, and the upper shell being flat. Can you tell by examining an oyster shell which was the lower valve? Does it show signs of having been attached to the bottom? The young oyster, like the young mussel, is free-swimming. Like the arthropoda, most mollusks undergo a metamorphosis to reach the adult stage (Fig. 199).

[Illustration: FIG. 199.--OYSTER.

_C_, mouth; _a_, vent; _g_, _g′_, ganglia; _mt_, mantle; _b_, gill.]

[Illustration: FIG. 200.--TROCHUS.]

[Illustration: FIG. 201.--CYPRÆA. (Univalve, with a long opening to shell.)]

Examine the shells of clams, snails, scallops, and cockles. Make drawings of their shells. The slug is very similar to the snail except that it has no shell. If the shell of the snail shown in Fig. 202 were removed, there would be left a very good representation of a slug.

=Economic Importance of Mollusca.=--Several species of clams are eaten. One of them is the _hard-shell clam_ (quahog) found on the Atlantic coast from Cape Cod to Texas. Its shell is white. It often burrows slightly beneath the surface. The _soft-shell clam_ is better liked as food. It lives along the shores of all northern seas. It burrows a foot beneath the surface and extends its siphons through the burrow to the surface when the tide is in, and draws into its shell the water containing animalcules and oxygen.

_Oysters_ to the value of many millions of dollars are gathered and sold every year. The most valuable oyster fisheries of the United States are in Chesapeake Bay. The young oysters, or “spat,” after they attach themselves to the bottom in shallow water, are transplanted. New oyster beds are formed in this way. The beds are sometimes strewn with pieces of rock, broken pottery, etc., to encourage the oysters to attach themselves. The dark spot in the fleshy body of the oyster is the digestive gland, or liver. The cut ends of the tough adductor muscles are noticeable in raw oysters. The starfish is very destructive in oyster beds.

_Pearls_ are deposited by bivalves around some irritating particle that gets between the shell and the mantle. The pearl oyster furnishes most of the pearls; sometimes pearls of great value are obtained from fresh-water mussels in the United States. Name articles that are made partly or wholly of mother-of-pearl.

[Illustration: FIG. 202.--A SNAIL.

_l_, mouth; _vf_, _hf_, feelers; _e_, opening of egg duct; _fu_, foot; _ma_, mantle; _lu_, opening to lung; _a_, vent.]

=Study of a Live Snail or Slug.=--Is its body dry or moist? Do land snails and slugs have lungs or gills? Why? How many pairs of tentacles has it? What is their relative length and position? The eyes are dark spots at bases of tentacles of snail and at the tips of the rear tentacles of slug. Touch the tentacles. What happens? Do the tentacles simply stretch, or do they turn inside out as they are extended? Is the respiratory opening on the right or left side of the body? On the mantle fold or on the body? (Figs. 202-3-4.) How often does the aperture open and close?

[Illustration: FIG. 203.--A SLUG.]

[Illustration: FIG. 204.--CIRCULATION AND RESPIRATION IN SNAIL.

_a_, mouth; _b_, _b_, foot; _c_, vent; _d_, _d_, lung; _h_, heart.

Blood vessels are black. (Perrier.)]

Place the snail in a moist tumbler. Does the whole under surface seem to be used in creeping? Does the creeping surface change shape as the snail creeps? Do any folds or wrinkles seem to move either toward the front or rear of its body? Is enough mucus left to mark the path traveled? The fold moves to the front, adheres, and smooths out as the slug or snail is pulled forward.

=Cephalopods.=--The highest and best developed mollusks are the =cephalopods=, or “head-footed” mollusks. Surrounding the mouth are eight or ten appendages which serve both as feet and as arms. These appendages have two rows of sucking disks by which the animal attaches itself to the sea bottom, or seizes fish or other prey with a firm grip. The commonest examples are the _squid_, with a long body and ten arms, and the _octopus_, or devilfish, with a short body and eight arms. Cephalopods have strong biting mouth parts and complex eyes somewhat resembling the eyes of backboned, or vertebrate, animals. The large and staring eyes add to the uncanny, terrifying appearance.

[Illustration: FIG. 205.--A SQUID.]

The sepia or “ink” discharged through the siphon of the squid makes a dark cloud in the water and favors its escape from enemies almost as much as its swiftness (Fig. 205). The squid sometimes approaches a fish with motion so slow as to be imperceptible, and then suddenly seizes it, and quickly kills it by biting it on the back behind the head.

The octopus is more sluggish than the squid. Large species called devilfish sometimes have a spread of arms of twenty-five feet. The _pearly nautilus_ (Fig. 206) and the _female of the paper argonaut_ (Fig. 207) are examples of cephalopods that have shells. The _cuttlefish_ is closely related to the squid.

[Illustration: FIG. 206.--PEARLY NAUTILUS. (Shell sawed through to show chambers used when it was smaller, and siphuncle, _S_, connecting them. Tentacles, _T_.)]

[Illustration: FIG. 207.--PAPER ARGONAUT (female).

× ¹⁄₃ (_i.e._ the animal is three times as long and broad as figure).]

[Illustration: FIG. 208.--PAPER ARGONAUT (male). × ¹⁄₂.]

=General Questions.=--The living parts of the mussel are very soft, the name mollusca having been derived from the Latin word _mollis_, soft. Why is it that the softest animals, the mollusks, have the hardest coverings?

To which class of mollusks is the name acephala (headless) appropriate? Lamellibranchiata (platelike gills)?

Why is a smooth shell suited to a clam and a rough shell suited to an oyster? Why are the turns of a snail’s shell so small near the center?

Why does the mussel have no use for head, eyes, or projecting feelers? In what position of the valves of a mussel is the hinge ligament in a stretched condition? How does the shape of the mussel’s gills insure that the water current and blood current are brought in close contact?

The three classes of mollusks are: the pelecypoda (hatchet-footed); gastropoda (stomach-footed); and cephalopoda (head-footed). Give an example of each class.

Comparison of Mollusks.

=====+==========+==========+========== | MUSSEL | SNAIL | SQUID -----+----------+----------+---------- | | | Shell| | | | | | -----+----------+----------+---------- | | | Head | | | | | | -----+----------+----------+---------- | | | Body | | | | | | -----+----------+----------+---------- | | | Foot | | | | | | -----+----------+----------+---------- | | | Gills| | | | | | -----+----------+----------+---------- | | | Eyes | | | | | | =====+==========+==========+==========

=Comparative Review.=--(To occupy an entire page in notebook.)

============+==========+==========+==========+==========+========== | GRASS- | | | | | HOPPER | SPIDER | CRAYFISH | CENTIPEDE| MUSSEL ------------+----------+----------+----------+----------+---------- | | | | | Bilateral | | | | | or radiate | | | | | ------------+----------+----------+----------+----------+--------- Appendages | | | | | for | | | | | locomotion | | | | | ------------+----------+----------+----------+----------+--------- Names of | | | | | divisions | | | | | of body | | | | | ------------+----------+----------+----------+----------+--------- Organs and | | | | | method of | | | | | breathing | | | | | ------------+----------+----------+----------+----------+--------- | | | | | Locomotion | | | | | | | | | | ============+==========+==========+==========+==========+=========