Fig. 62.—Bullia laevissima Gmel., showing branchial siphon S; F, F, F, foot; OP, operculum; P, penis; Pr, proboscis; T, T, tentacles. (After Quoy and Gaimard.)
In the great majority of marine Prosobranchiata the branchia is securely concealed within a chamber or pouch (the respiratory cavity), which is placed on the left dorsal side of the animal, generally near the back of the neck. For breathing purposes, water has to be conveyed into this chamber, and again expelled after it has passed over the branchia. In the majority of the vegetable-feeding molluscs (e.g. Littorina, Cerithium, Trochus) water is carried into the chamber by a simple prolongation of one of the lobes or lappets of the mantle, and makes its exit by the same way, the incoming and outgoing currents being separated by a valve-like fringe depending from the lobe. In the carnivorous molluscs, on the other hand, a regular tube, the branchial siphon, which is more or less closed, has been developed from a fold of the mantle surface, for the special purpose of conducting water to the branchia. After performing its purpose there, the spent water does not return through the siphon, but is conducted towards the anus by vibratile cilia situated on the branchiae themselves. In a large number of cases, this siphon is protected throughout its entire length by a special prolongation of the shell called the canal. Sometimes, as in Buccinum and Purpura, this canal is little more than a mere notch in the ‘mouth’ of the shell, but in many of the Muricidae (e.g. M. haustellum, tenuispina, tribulus) the canal becomes several inches long, and is set with formidable spines (see Fig. [164], p. 256). In Dolium and Cassis the canal is very short, but the siphon is very long, and is reflected back over the shell.
The presence or absence of this siphonal notch or canal forms a fairly accurate indication of the carnivorous or vegetarian tendencies of most marine Prosobranchiata, which have been, on this basis, subdivided into Siphonostomata and Holostomata. But this classification is of no particular value, and is seriously weakened by the fact that Natica, which is markedly ‘holostomatous,’ is very carnivorous, while Cerithium, which has a distinct siphonal notch, is of vegetarian tendencies.
In the Zygobranchiata the water, after having aerated the blood in the branchiae, usually escapes by a special hole or holes in the shell, situated either at the apex (Fissurella) or along the side of the last whorl (Haliotis). In Pleurotomaria the slit answers a similar purpose, serving as a sluice for the ejection of the spent water, and thus preventing the inward current from becoming polluted before it reaches the branchiae (see Fig. [179], p. 266).
In Patella the breathing arrangements are very remarkable. In spite of their apparent external similarity, this genus possesses no such symmetrically paired plume-shaped branchiae as Fissurella, but we notice a circlet of gill-lamellae, which extends completely round the edge of the mantle. It has been shown by various authorities that these lamellae are in no sense morphologically related to the paired branchiae in other Mollusca, but only correspond to them functionally. The typical paired branchiae, as has been shown by Spengel, exist in Patella in a most rudimentary form, being reduced to a pair of minute yellow bodies on the right and left sides of the back of the ‘neck.’ A precisely similar abortion of the true branchiae, and special development of a new organ to perform their work, is shown in Phyllidia and Pleurophyllidia (see below under [Opisthobranchiata]). This circlet of functional gills in Patella has therefore little systematic value, being only developed in an unusual position, like the eyes on the mantle in certain Pelecypoda, to supply the place of the true organs which have fallen into disuse. Accordingly Cuvier’s class of Cyclobranchiata, which included Patella and Chiton, has no value, and has indeed long been discarded. In Chiton the gills never extend completely round the animal, but are always more or less interrupted at the head and anus. They are the true gills, the plumes being serially repeated in the same way as the shell plates.
Fig. 63.—Patella vulgata L., seen from the ventral side: f, foot; g.l, circlet of gill lamellae; m.e, edge of the mantle; mu, attachment muscle; sl, slits in the same; sh, shell; v, vessel carrying aerated blood to the heart; v´, vessel carrying blood from the heart; ve, small accessory vessels.
Fig. 64.—Patella vulgata L., seen from the dorsal side after the removal of the shell and the black pigment covering the integument; the anterior portion of the mantle is cut away or turned back: a, anus; br, br, remains of the true branchiae (ctenidia); i, intestine; k, k´, kidneys; k.ap, their apertures on each side of the anus; l, liver; m, m, mantle; mu, attachment muscles, severed in removal of shell; t, t, tentacles.