228 HIGH SCHOOL ZOOLOGY. 



The sub-kingdom derives its name from the softness of the 

 tissues of the body, a peculiarity dependent on the fact that it 

 is generally protected by a one or two-valved shell. Locomo- 

 tion is effected by the specialised musculatui'e of the ventral 

 surface of the body — the so-called foot — which recalls, in many 

 cases, the creeping surfaces of the Planarians, but is often 

 curiously modified for other methods of locomotion. The i^e- 

 spii-atory organs are generally gills, situated on one or both sides 

 of the body and protected by a fold of the skin called the 

 pallium or mantle ; it is this portion of the skin which has the 

 function of seci'eting the shell. One jiair of excretory organs, 

 similar in structure to the nephridia of the segmented worms, 

 are piesent, but the nervous system is arranged on a plan 

 entirely different from that of any of the Vermes studied. 



15. Two subdivisions of Molluscs are recognized, which differ 

 according to the amount of specialisation of the cephalic end of 

 the body. The bivalve shell-fish, from the peculiar manner in 

 which their food is secured and their almost sedentary habits, 

 have none of that concentration of the sense-organs and of the 

 nerve-centres into the " head," which we find in the other 

 Molluscs. They are thus frequently called the Acephala, in con- 

 trast to the Cephalophora. 



16. The Acephala form a single class — Lamellibranchiata, 

 called so on account of the plate-like gills, which are present in 

 all. It is chiefly a marine group, but representatives of both of the 

 orders into which it is subdivided occur in fresh water, the most 

 conspicuous being the fresh-water Mussels (Unionidce), any 

 one of which will serve as a type for the study of the class. 



The shell, like the body, is symmetrical, the right and left 

 valves being similar ; it is only in attached forms of Lamelli- 

 branchs like the oyster, in which any great degree of asymmetry 

 is to be observed. At the dorsal surface is to be noted the 

 hinge, formed by an an uncalcified part of the shell, the ligament, 



