1^'^7.] NATURAL SCIEXCES OF I'HI LADKLPFilA. 343 



the shell <;lii"«l. which develops in the inverse order of that on the 

 dorsal side (t(>\t figure). In the stages shown in figs. 30 and 31 the 

 cerohial ganglia lie lateral and slightly posterior to the blastopore; 

 in fig. 32 they lie on the sitlcs of the blastopore, and a process is growing 

 out from each toward the other in front of the blastopore; in fig. 33-36 

 they lie in front of the blastopore and are closely connected together 

 by the cerebral conunissurc. 



Along the posterior edge of each cerebral ganglion a leaf-like process 

 is formed which is the tentacle (figs. 35, 36, T.), and at the outer end of 

 the furrow which separates the tentacle from the blastoderm the eye 

 appears (figs. 34-36). 



h. Buccal Ganglia. — Lateral and slightly posterior to each cerebral 

 anlage is a group of cells which gives rise to the buccal ganglion (figs. 

 22, 23, Be). These cells lie at the very periphery of the blastoderm 

 and are probably derived from the ectomeres 2a and 2c. In the over- 

 growth of the yolk they accompany the cerebral ganglia, moving from 

 the dorsal to the ventral side in the posterior lip of the blastopore 

 (figs. 27-29), and finally coming to lie along the posterior border of 

 the blastopore (fig. 30). A\lien the blastopore closes the buccal ganglia 

 of the two sides come into contact on the posterior side of the blasto- 

 pore (fig. 31 and text figure). 



c. Pleural, Pedal, Parietal and Abdominal Ganglia. — ^The other 

 ganglia of the nervous system form part of the complex of organs lying 

 between the shell gland and the posterior lip of the blastopore. The 

 l)leural ganglia lie on each side of the shell gland and on the lateral 

 borders of the organ complex (figs. 23, 24) ; the pedal ganglia lie on the 

 median side of the pleurals; close behind the shell gland are the 

 groups of cells which give rise to the parietal and abdominal ganglia. 

 The anlagen of these ganglia are not clearly separated at this time, and 

 only in the later stages (fig. 28 cl seq.) are they quite distinct. When 

 the foot begins to appear (fig. 31) the pedal ganglia are included 

 within it. and the pleural ganglia lie on each side of the foot, while the 

 otocysts lie between the two (fig. 28 et seq.). The otocysts are here, as 

 elsewhere, formed as an invagination of the superficial ectoderm. 

 All the ectodermal organs of this region are derived from the cell 2d, 

 with the possible exception of the pleural ganglia, which lie on the 

 lateral borders of the organ complex and may therefore come, in part, 

 from the cells 3c and Sd. In aimelids the ectoderm of the trunk region 

 and the ventral neural plate, which gives rise to all the nervous system 

 posterior to the mouth, come from this same cell, 2d, the "first somato- 

 blast." Lillie (1895) fo\md the same condition among ]amellil)ranchs. 



