Fig. 14. Bulla ampulla.
Order III.—Opisthobranchiata. These animals carry their gills exposed on the back and sides, towards the rear of the body. Only a few have any shell. 1. The Tornatellidæ, which have a stout little spiral shell. 2. The Bullidæ, in which the spire is concealed ([Fig. 14]). 3. The Aplysiadæ, where the shell is flat and oblong or triangular in shape. The remaining families are slug-like and shell-less.
Order IV.—Nucleobranchiata. Derives its name from the fact that the animals constituting it have their respiratory and digestive organs in a sort of nucleus on the posterior part of the back, and covered by a minute shell. As they are pelagic, the shells are not readily to be obtained. They are divided into two families, Firolidæ and Atlantidæ.
CLASS III.—PTEROPODA. Like the last, these pretty little mollusca are ocean-swimmers. The members of one division of them, to which the Cleodora belongs, is furnished with iridescent external shells.
CLASS IV.—The LAMELLIBRANCHIATA (Plate-gilled), or Conchifera (Shell-bearing), includes the mollusca commonly known as "bivalves," the animal being snugly hidden between two more or less closely fitting shelly valves. The oysters, cockles, etc., are examples of this class. 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 shell on which the teeth and ligament are situated is termed the hinge-line. A bivalve is said to be equivalve when the two shells composing it are of the same size, inequivalve when they are not. If the umbones are in the middle, the shell is equilateral ([Fig. 15]); but inequilateral when they are nearer one side than the other ([Fig. 16]). If the shell be an oyster or a scallop, you will find on the inside a single circular scar-like mark near the
Fig. 15. Petunculus guerangeri.
Fig. 16. Venus plicata. 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 shell. The former group on this account are often called Monomyaria (having one shell-muscle), and the latter Dimyaria (having two shell-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 shell ([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 shell. It marks the line of attachment of the mantle or shell-secreting organ of the animal to the shell 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 shell, 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 shell is closed.
Attention to these particulars is necessary when arranging your bivalves, as on them their classification depends, the class being divided into—