IX 



MOLLUSCA 



299 



grows out as a narrow tube. The latter comes into contact with 

 the ectoderm just behind a slight ventral projection carrying stiff 

 hairs, which now becomes apparent and which corresponds to the 

 telotroch of the larva of Polygordius: there the anus is formed 

 later. There is, in addition to the telotroch, a mid-ventral band of 

 very fine cilia passing forwards from the telotroch to the mouth, it 

 corresponds to the mid-ventral ciliated groove found in many Anne- 

 lid larvae, although it is not found in the larva of Polygordius. 



There are, however, several points in which the larva has already 

 passed the stage of the 

 pure Trochophore larva 

 and entered on a post- 

 trochophoral stage of de- 

 velopment. These are as 

 follows: (1) In the dorsal 

 region, behind the proto- 

 troch, a thickened plate of 

 ectoderm cells has become 

 recognizable, which is 

 beginning to be slightly 

 invaginated; this is the 

 rudiment of the shell 

 gland, the organ to which 

 the shell owes its origin. 

 (2) The mother cells of 

 the adult mesoderm have 

 proliferated, each giving 

 rise to a short mesodermal 

 band. (3) Two slight 

 elevations at either side 

 of the mid -ventral line 

 have appeared, just behind 

 the mouth, which are the 

 beginnings of that char- 

 acteristic Molluscan organ, 

 the foot. 



As development pro- 

 ceeds the life -history of 

 the Mollusc diverges more and more widely from that of the Annelid. 

 In the case of Patella the post-trochophoral stages of development 

 have been described by Patten (1885); he was able to keep the 

 larvae alive for a week, and has given a good account not only of 

 the external appearance of the larva in these stages, but also, to 

 some extent, of their structure, by means of sections. 



Patten's account, although extremely interesting, is rather meagre, 

 and whets the desire for a thorough reinvestigation ; it is as follows. 

 As the blastopore shifts forward and finally closes near the spot where 

 the mouth will subsequently be formed, the two rudiments of the 



FIG. 234. Lateral view of young Trochophore larva 

 of Patella coerulea. (After Wilson, slightly 

 altered. ) 



ap, apical plate ; M, mother cell of mesoblast ; m.b, meso- 

 dermal band ; s.g, shell gland ; st, stomach ; stom, stomo- 

 daeum. 



