26 



COMPARATIVE ANATOMY 



Fiii. 

 Mollusc, diagraininatic, left aspect. 

 Mouth ; 1:, head ; sm, shell iiniscle ; oso, 

 upper aperture of the shell ; o, anus ; /(, 



I. Organisation of the Primitive Mollusc. 



The hypothetical primitive Molkisc, reconstructed from the results 

 of morphological research, may be described as follows : — 



The body is bilaterally symmetrical and dorsally arched ; its 

 anterior end carries the mouth, eyes, and 

 tentacles, forming a distinct head. The 

 ventral side forms a powerful muscular 

 foot, distinct from the rest of the body, 

 with a flat sole for creeping. 



The soft integument of the arched 

 dorsal side forms a fold, which hangs 

 down all round the body, and is called 

 Hypothetical Primitive the mantle or pallium. The mantle 

 encloses a circular cavity, the mantle- 

 or pallial cavity, which surrounds the 



renal aperture; ,.h, mantle cavity; ct, ^^^ ^^^^^ COmmUuicateS freely with the 

 ctenioiuni ; /, foot. "^ -, . i c 



surrounding medium between the free 

 edge of the mantle and the foot. The dorsal integument of the body 

 and of the mantle secretes a closely-ai^plied shell, which consists of a 

 chitinous matrix (conchyolin) in- 

 ter-stratified with deposits of car- 

 bonate of lime. This shell repeats 

 the form of the dorsal surface, and 

 is thus bilaterally symmetrical and 

 arched. Such a shell detached and 

 turned over would resemble a cup 

 or plate. Since the dorsal shell 

 covers the whole, or at any rate 

 the greater part of the body, it 

 forms a protection for it and at 

 the same time plays the part of a 

 skeleton, to which the muscles run- 

 ning more or less dorso-ventrally 

 into the foot and head, can be 

 firmly attached. 



The mantle is of special im- 

 portance as a protective structure. fig. 39.— Hypothetical Primitive Mollusc, 

 Apart from the fact that its edge fro'" above. 0, Moutli ; uh-, nlpl, nip, primitive 



secretes the greater part of the shell '''' '"''t-.fieffa.'rli'hJlrreJf UnSi' 



o _ i_ urpa, primitive left and right parietal ganglia, 



substance, and in this way adds to 



rpa, pri 

 ula, primitive left auricle ; vos, uros 



primitive 



the shell as the animal grows, it ^eft and right osphradia (Spcngel's organ) ; vlct, 



urct, primitive left and right ctenidia (gills) ; mb, 

 base of the mantle ; inr, edge of the mantle ; m, 



covers the delicate gills, which 

 thus also share the protection 



lantle cavity ; v, visceral ganglion ; ve, ventricle ; 



afforded by the shell. Analogous "> ''^""*^' 



arrangements are to be found in other divisions of the animal kingdom, 



