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 

 PIG. 3.s. -Hypothetical Primitive the man tle or pallium. The mantle 



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



Mouth; fc, head; sm, shell muscle ;osoi encloses a Circular Cavity, the mantle- 

 upper aperture of the shell; a, anus; m, or pallial cavity, which surrounds the 



renal aperture; mil, mantle cavity; c.t, ^ an( j commun j ca t e s freely With the 

 cteniilinin ; /, foot. it 



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-ventrall}' 

 into the foot and head, can be 

 firmly attached. 



The mantle is of special im- 

 portance as a protective Structure. 

 Apart from the fact that its edge 



pic. 3H. Hypothetical Primitive Mollusc. 

 m "bove. o, Mouth : ?..\ ul t ,l, //-. primitive 

 ,1 F .-, i 11 left cerebral pleural and pedal ganglia; ulpa, 



secretes the greater part of the shell urpa> prim ' iti / e left and right parietal , a]ldi: , . 



Substance, and in this Way adds to nl, primitive U-ft auricle ; uos, wros, primitive 



the Shell as the animal grOWS, it left 



, , ,. ... *- 



covers the delicate glllS, 



tllUS also share the protection mantle cavity ; c. visceral ganglion; re, ventricle; 



afforded by the shell. Analogous "' anus - 



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



ri S ht l'l>ra.lia (Spengel's organ) ; Oat, 

 ,. , urct, primitive left and right ctenidia (gills) ; ml), 

 which base O f the mantle ; mr, edge of the mantle ; TO, 



