26 



COMPARATIVE ANATOMY 



CHAP. 



03 



renal aperture; mh, mantle cavity; 

 ctenidium ; /, foot. 



I. Organisation of the Primitive Mollusc. 



The hypothetical primitive Mollusc, 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 

 PIO. 38.- Hypothetical Primitive fche man tle or pallium. The mantle 



Mollusc, diagrammatic, left aspect, o, -, , , , 



Mouth; t, head;. <m, shell muscle; 0*0, encloses a ClTCUlaT cavity, the mantle- 

 upper aperture of the shell ; o, anus; /,, or pallial Cavity, which SUrrOUllds the 



bod and communicates freely with the 



i < 



surrounding medium between the tree 

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

 and of the mantle secretes a closely-applied 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. F I(J . 39. Hypothetical Primitive Mollusc, 

 Apart from the fact that its edge from above. , Mouth ; vlf, M vlp, primitive 



SPcrptM thp o-rpqfpr narr nf rtiP hpll left cerebral P 1<>ural and P e ' lal &*&*'> ^"' 

 16 greater part I urpa ^ primitive left and right parietal ganglia; 



Substance, and in this Way adds tO ula, primitive left auricle ; uos, uros, primitive 



the Shell as the animal STOWS, it left and right osphradia (S^ngel's organ) ; Zcf, 



,i JT ... 1-1 urct, primitive left and right ctenidia (gills) ; mb, 



covers the delicate gills, which base of the raantle . mr> edge of the mant i e ; m, 



thus also share the protection mantle cavity ; i; visceral ganglion ; vc, ventricle ; 



afforded by the shell. Analogous a ' anus - 



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



