248 



LOWER INVERTEBRATES. 



Branch VII. — MOLLUSCA. 



With the possible exceptions of the insects and the birds, tliere is no group in the 

 animal kingdom which is such an universal favorite among all classes as the one now 

 under discussion. This is very natural; for the hard armor which they bear, and the 

 bright colors with which many of them are ornamented, renders them attractive, while 

 the comparative indestructibility of the same shells renders the care of a collection an 

 easy task. But while the collectors of the shells are many, the real students of the 

 animals are few, and even now, although these forms have been collected and studied 

 by conchologists for many years, a satisfactory classification is still desired. 



The word Mollusca means soft, and it was applied by Linne to a group of animals 

 embracing of the true molluscs only the naked forms, together with the hydroids, 

 echinoderms and annelids, while the shell-bearing molluscs were arrangied as Testacea 

 in a section of his grouj) of Vermes. Cuvier was the fii'st to introduce order into the 

 group. His studies during the seven years spent as tutor on the Normandy coast 

 resulted in a classification of the Mollusca upon truly scientific grounds. The gi-oup, 

 as recognized by him, embraced, besides the forms now admitted, the barnacles, the 

 ascidians, and the brachiopods, truly a heterogeneous assemblage. In after years the 

 Polyzoa were drawn in. The first of these groups to be separated were the barnacles, 

 which were shown by Thompson to be Crustaceans in 1831. Then Kowalewsky, in 

 1865, described the embryology of the ascidians, from which it was apparent that they 

 had no relationships with the molluscs, but were rather to be classed with the verte- 

 brates, and lastly, the brachiopods were absolutely divorced from the group, taking the 

 Polyzoa with them. 



A concise definition of the Mollusca is impossible. Here, as elsewhere, nature 

 refuses to be bound by strict rules, and the best we can do is to form a general concep- 

 tion which shall be true of the majority of forms, and which will, at the same time, be 

 loose enough to admit all. A mollusc, then, is a bilaterally symmetrical, unsegmented 

 animal, usually covered with a univalve or bivalve shell. It has a ventral, muscular 

 portion (the foot) well developed ; a symmetrical nervous system, consisting of a 



brain or supra-oesophageal ganglia, an oeso- 

 phageal commissure, and a secondary brain 

 beneath the throat. Most forms, in their 

 development, j)ass through a trochozoon 

 stage. 



In some forms, especially in the gastero- 

 poda, the bilateral symmetry of the body is 

 more or less obscured, owing to what may 

 be called a torsion of the body, but, never- 

 theless, if ^\■e make allowance for this twist- 

 ing, it can readily be traced. In the young. 



Fig. 253 — Bjagram of mollusc, a, anus, b, brain, cere- 

 bral ganglion, fj foot, Oj genital opening, h, heart; 

 i, pleural ganglion; /, liver; m, moutli; ?i, kidney; 

 Pf pedal ganglion; v, visceral ganglion. 



the segmentation of the body is frequently evident, but it entirely disappears in the 

 adults, except among the chitons, where the elements of the shell and the gills are 

 metamerically repeated. 



The foot is a muscular process on the lower surface of the body, which is highly 



