Part 4
[Ill.u.s.tration: Fig. 15. _Petunculus guerangeri._]
[Ill.u.s.tration: Fig. 16. _Venus plicata._]
CLa.s.s IV.--The LAMELLIBRANCHIATA (Plate-gilled), or CONCHIFERA (Sh.e.l.l-bearing), includes the mollusca commonly known as "bivalves,"
the animal being snugly hidden between two more or less closely fitting sh.e.l.ly valves. The oysters, c.o.c.kles, etc., are examples of this cla.s.s. The two valves are fastened together near their points, or beaks (technically called _umbones_), by a tough elastic ligament, sometimes supplemented by an internal cartilage. If this be severed and the valves parted, it will be found that in most cases they are further articulated by projecting ridges or points called the _teeth_, which, when the valves are together, interlock and form a hinge; the margin of the sh.e.l.l on which the teeth and ligament are situated is termed the _hinge-line_. A bivalve is said to be _equivalve_ when the two sh.e.l.ls composing it are of the same size, _inequivalve_ when they are not. If the umbones are in the middle, the sh.e.l.l is _equilateral_ (Fig. 15); but _inequilateral_ when they are nearer one side than the other (Fig. 16). If the sh.e.l.l be an oyster or a scallop, you will find on the inside a single circular scar-like mark near the centre; this is the point to which the muscles that close the valves and hold them so tightly together are attached. In the majority of bivalves, however, there are two such muscular impressions, or scars, one on either side of each valve of the sh.e.l.l. The former group on this account are often called _Monomyaria_ (having one sh.e.l.l-muscle), and the latter _Dimyaria_ (having two sh.e.l.l-muscles). In the last named the two muscular impressions are united by a fine groove (or _pallial-line_), which in some runs parallel to the margin of the sh.e.l.l (Fig. 15), whilst in others it makes a bend in (_pallial-sinus_) on one side of the valve towards the centre (Fig. 16). In Monomyaria it will be found running parallel to the margin of the sh.e.l.l. It marks the line of attachment of the mantle or sh.e.l.l-secreting organ of the animal to the sh.e.l.l which grows by the addition of fresh matter along its edges, so that the concentric curved markings so often seen on the exterior correspond in their origin with the periodic mouths of the Gasteropods. The bivalves are all aquatic, and many bury themselves in the sand or mud by means of a fleshy, muscular foot. These are furnished with two siphons, or fleshy tubes, sometimes united, sometimes separate, through which they respire, drawing the water in through one and expelling it by the other. Those kinds whose habit it is to bury themselves below the surface of the mud or sand are furnished with long retractile siphons, and to admit of their withdrawal into the sh.e.l.l, the mantle is at this point attached farther back, giving rise to the _pallial-sinus_ above described; this sinus is deeper as the siphons are proportionately longer, and in many cases, too, the valves do not meet at this point when the sh.e.l.l is closed.
Attention to these particulars is necessary when arranging your bivalves, as on them their cla.s.sification depends, the cla.s.s being divided into--
_a._ ASIPHONIDA (Siphonless).
_b._ SIPHONIDA _Integro-pallialia_ (with Siphons).--Pallial-line entire.
_c._ SIPHONIDA _Sinu-pallialia_ (with Siphons).--Sinus in pallial-line.
DIVISION _a_.--ASIPHONIDA--is next subdivided into--
1. The _Ostreidae_, or oysters, which are deservedly a distinct family in themselves.
2. The _Anomiadae_, comprising the multiform and curiously constructed _Anomia_, with the "Window-sh.e.l.ls" (_Placuna_).
3. The _Pectinidae_, taking its name from the genus _Pecten_, or "Scallop-sh.e.l.ls," of which one kind (_P. maximus_) is frequently to be seen at the fishmongers' shops. The "Thorney oysters" (_Spondylus_) take rank here, and are highly esteemed by collectors, one specimen indeed having been valued at 25!
4. The _Aviculidae_, or "Wing-sh.e.l.ls," among which are numbered the "Pearl-oyster" of commerce (_Meleagrina margaritifera_). The strange T-shaped "Hammer oyster" belongs to this family, as does also the _Pinna_. The Pinnas, like the mussels and some other bivalves, moor themselves to rocks by means of a number of threads spun by the foot of the mollusc, and termed the _byssus_, which in this genus is finer, more silky, than in any other, and has been woven into articles of dress.
5. The _Mytilidae_, or mussels, including the _Lithodomus_, or "Date-sh.e.l.l," which bores into corals and even hard limestone rocks.
[Ill.u.s.tration: Fig. 17. Hinge-teeth of _Arca barbata_.]
6. The _Arcadae_, or "Noah's-ark-sh.e.l.ls," characterized by their long straight hinge-line set with numerous very fine teeth (Fig. 17). The "Nut-sh.e.l.l" (_Nucula_) belongs to this family.
7. The _Trigoniadae_, whose single living genus, the handsome _Trigonia_ (Fig. 18), is confined to the Australian coast-line, whereas in times now long past they had a world-wide distribution.
[Ill.u.s.tration: Fig. 18. _Trigonia margaritacea._]
8. The _Unionidae_, comprising the fresh-water mussels.
DIVISION _b_.--SIPHONIDA _Integropallialia_.
1. The _Chamidae_, represented by the reef-dwelling _Chama_.
2. The _Tridacnidae_, whose sole genus _Tridacna_ contains the largest specimen of the whole cla.s.s of bivalves, the sh.e.l.ls sometimes measuring two feet and more across.
3. The _Cardiadae_, or c.o.c.kles.
4. The _Lucinidae_, in which the valves are nearly circular, and as a rule not very attractive in appearance, though the "Basket-sh.e.l.l"
(_Corbis_) has an elegantly sculptured exterior.
5. The _Cycladidae_, whose typical genus _Cyclas_, with its round form and thin h.o.r.n.y sh.e.l.l, is to be found in most of our ponds and streams.
6. The _Astartidae_, a family of sh.e.l.ls having very strongly developed teeth, and the surface of whose valves is often concentrically ribbed.
[Ill.u.s.tration: Fig. 19. Hinge of _Cardita sinuata_.]
7. The _Cyprinidae_, which have very solid oval or elongated sh.e.l.ls and conspicuous teeth (Fig. 19). The "Heart-c.o.c.kle" (_Isocardia_) belongs to this family.
DIVISION _c_.--SIPHONIDA _Sinu-pallialia_.
1. The _Veneridae_. The hard, solid sh.e.l.ls of this family are for elegance of form and beauty of colour amongst the most attractive a collector can posses. Their sh.e.l.ls are more or less oval and have three teeth in each valve (Fig. 20).
[Ill.u.s.tration: Fig. 20. Hinge of _Cytherea crycina_.]
[Ill.u.s.tration: Fig. 21. Hinge of _Lutraria elliptica_]
2. The _Mactridae_ are somewhat triangular in shape, and may be at once recognised by the pit for the hinge-ligament, which also a.s.sumes that form, as seen in the accompanying figure of _Lutraria elliptica_ (Fig. 21).
3. The _Tellinidae_ comprise some of the most delicately tinted, both externally and internally, of all sh.e.l.ls. In some, coloured bands radiate from the umbones, and well bear out the fanciful name of "Sunset sh.e.l.ls" bestowed upon them. Their valves are generally much compressed.
4. The _Solenidae_, or "Razor-sh.e.l.ls," rank next, and are readily recognised by the extreme length of the valves in proportion to their width, and also by their gaping at both ends.
5. The _Myacidae_ or "Gapers," have the siphonal ends wide apart (in the genus _Mya_ both ends gape), and are further characterized by the triangular process for the cartilage, which projects into the interior of the sh.e.l.l. One valve (the left) is generally smaller than the other.
6. The _Anatinidae_ have thin, often inequivalve pearly sh.e.l.ls. The genus _Pandora_ is the form most frequently met with in collections.
7. The _Gastrochaenidae_ embraces two genera (_Gastrochaena_ and _Saxicava_) of boring mollusca, which perforate sh.e.l.ls and rocks, and also, the remarkable tube-like "Watering-pot-sh.e.l.l" (_Aspergillum_) which is hardly recognisable as a bivalve at all.
[Ill.u.s.tration: Fig. 22. _Pholas dactylus._]
8. The _Pholadidae_ concludes the list of bivalves, and comprises the common rock-boring Pholas (Fig. 22) of our coasts and the wood-boring shipworm "Teredo" (Fig. 23).
Although the _Brachiopoda_, or "Lamp-sh.e.l.ls," are not true mollusca, they are not very far removed from them, and are so often to be found in cabinets that it will not do to pa.s.s them over, especially since in past times they were very abundant, an enormous number occurring in the fossil state. Only eight genera are now living. Sh.e.l.ls belonging to this cla.s.s are readily recognised by the fact of one valve being larger than the other, and possessing a distinct peak, the apex of which is perforated. The _Terebratulidae_ are the most extensive family of this cla.s.s.
[Ill.u.s.tration: Fig. 23. _Teredo navalis._]
HOW TO ARRANGE THE Sh.e.l.lS IN THE CABINET.
When you have arranged your specimens in the order above indicated, proceed to place them in your boxes, arranging and labelling them after the manner shown in the accompanying diagram.
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Cla.s.s.
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Generic
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Generic
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