26 



COMPARATIVE ANATOMY 



CHAP. 



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 

 the mantle or pallium. The mantle 

 encloses a circular cavity, the mantle- 



renal aperture ; mJt, 

 cteniclium ; /, foot. 



lantle cavity; vt, 



FIG. 38. Hypothetical Primitive 

 Mollusc, diagrammatic, left aspect, o, 

 Mouth ; fc, head ; sm, shell muscle ; oso, 

 upper aperture of the shell; a, anus; n, or palllal Cavity, which Surrounds the 



body, and communicates freety with the 

 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-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. Fl(; . 39. -Hypothetical Primitive Mollusc, 



Apart from the fact that its edge from above, o, Mouth ; ulc, ?/>/, ulp, primitive 



secretes the greater part of the shell 

 substance, and in this way adds to 

 the shell as the animal grows, it 

 covers the delicate gills, which 

 thus also share the 

 afforded bv the shell. 



left cerebral pleural and pedal ganglia ; iilpa, 

 urpa, primitive left and right parietal ganglia ; 

 nla, primitive left auricle ; iios, uros, primitive 

 left and right osphradia (Spengel's organ) ; itlct, 

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

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

 protection mantle cavity ; r, visceral ganglion ; re, ventricle ; 



Analogous 



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



