IX 



MOLLUSCA 



537 



an ectodermic invagination just where the last trace of the blastopore 

 was situated. 



The wall of the mid-gut, after the blastopore has become closed, 

 undergoes a characteristic differentiation. The cells forming its 

 anterior wall acquire large clear nuclei with conspicuous nucleoU, 

 whilst those forming the lateral and posterior walls retain small 

 deeply staining nuclei. Soon the peculiar cells of the anterior wall 

 become confined to two slight outpouchings of the wall of the 

 stomach, to the right and the left of the mid-ventral line. These 

 pouches of the larval stomach will eventually give rise to the adult 

 liver. From the posterior wall of the stomach is developed the 

 intestine, and this grows 

 backwards and becomes 

 attached to the ectoderm 

 behind the mouth. Here a 

 very shallow invagination is 

 formed, the proctodaeum; 

 and at a slightly later stage, 

 by the union of the procto- 

 daeum and intestine, the anus 

 becomes opened. In front 

 of the anus is formed the 

 teloroch, consisting of a 

 couple of cells carrying stiff 

 hairs. 



So far we have not men- 

 tioned tbe prototroch and 

 the apical plate. Both 

 these structures appear about 

 the time when the shell 

 gland is everted ; the proto- 

 troch is in the form of a 

 girdle of cells carrying 

 powerful cilia, and the apical 

 plate in the form of a group 

 of cells at the animal pole bearing a wisp of long stiff cilia. The cell- 

 hneage of the cells forming these organs Meisenheimer was not able 

 to determine, but there is no reason to doubt that it is, in the mam, 

 the same as in Patella. The prototrochal cells develop vacuoles in 

 their interior, as is the case with the prototrochal cells of Polygordius. 

 Lastly, situated just behind the spot where the anus will develop, 

 there is a group of small cells which Meisenheimer believes to be of 

 ectodermal origin, which will give rise, at a later period, to the 

 coelomic sacs and to their derivatives, the kidneys and genital 

 organs. This cell-group occupies precisely the same place as does 

 the first rudiment of the pericardium in Paludina, and as do the 

 mother cells of the mesoderm in an earlier stage of development m 

 Dreissensia. When this stage of development has been attained, the 

 VOL. I ^ 



Fig. 266. — Young Trochophore larva of Dreis- 

 sensia polymorpha, seen from tbe ventral side. 

 (After Meisenheimer.) 



Letters as in previous figure. In addition, t.tr, telotrocli. 



