86 COMPARATIVE ANATOMY 



the blastopore which has closed up. The shell-gland appears 

 as a patch of columnar ectoderm on the dorsal surface opposite 

 to the foot, the patch being generally invaginated to a greater 

 or less extent. 



It is, however, in the subsequent phases of growth that the 

 Mollusc deviates most markedly from the Annelid plan of 

 development. One may attribute a large part of the diver- 

 gence to the behaviour of the mesoblast. Instead of elongat- 

 ing, thickening, and segmenting as in the Annelid, the 

 mesoblast bands of the Mollusc break up into a number of 

 amoeboid cells which insinuate themselves everywhere in the 

 space between the walls of the gut and the body wall. But 

 two more or less distinct cell masses remain at the posterior 

 end of the body, right and left of the anus in those cases in 

 which the posterior part of the blastopore persists as the anus. 

 These cell masses eventually become hollow and form a pair 

 of vesicles, whose cavities represent the true ccelom. Usually 

 the left vesicle degenerates while the right vesicle increases in 

 size and gives rise to the pericardium, the excretory organ, and 

 the reproductive organs. 



Meanwhile the growth of the shell has materially altered the 

 external aspect of the larva. The shell-gland is evaginated, 

 and gives rise to a larval shell, which covers the dorsal surface 

 like a cap and grows enormously in size relatively to the other 

 parts of the body. The larval shell is generally more or less 

 conical or helmet-shaped, and its rounded extremity curves 

 over towards the anterior aspect of the body. The mouth of 

 the shell is surrounded by a prominent thickening of the body- 

 wall, representing the mantle. As a result of the excessive 

 growth of the shell on the dorsal side, the anus, which was 

 originally terminal, becomes pushed over to the ventral surface, 

 and as growth proceeds it is pushed further and further for- 

 wards till eventually it lies not far behind the mouth, being 

 separated from the latter orifice by the foot, which has not as 

 yet undergone any considerable increase in size. As a con- 

 sequence of this approximation of the mouth and anus the 

 alimentary canal has been flexed in the manner shown in 

 fig. 20, ^Tandy, but the larva is still bilaterally symmetrical, and 

 there is no sign as yet of any torsion of the visceral mass 

 lodged in the shell. 



By this time several important structures have made their 



