344 PROCEEDINGS OF THE ACADEMY OF [Jl^'ly, 



Among the gasteropods this cell is not larger than the other mem- 

 bers of the second quartet, but it is destined to give rise to all the 

 important ectodermal organs posterior to the mouth. In this fact we 

 have a striking illustration not only of the value of cell-lineage in com- 

 parative embryology, but also of the fundamental similarity of 

 annelids, lamellibranchs and gasteropods. 



The parietal and abdominal ganglia, with their connectives, are 

 recognizable as thickenings of the blastoderm in a stage as early as 

 fig. 28, but they are not clearly distinguishable until a later stage 

 (figs. 30, 33, 34), In these figures the entire nervous system is easily 

 recognizable. The cerebral ganglia are connected by the cerebral com- 

 missure anterior to the mouth, and they are united with the pleurals 

 and pedals by the cerebro-pleural and the cerebro-pedal commissures 

 respectively on each side of the cesophagus. The parietal ganglion of 

 the left side lies almost directly behind the foot, and is connected with 

 the pleural of the left side by the pi euro-parietal connective; the 

 parietal of the right side lies on the right side of the foot and is connected 

 with the pleural of that side, while the two parietals are connected 

 together by the nerve loop which runs alongside of the intestine and 

 near its middle bears the abdominal, or visceral, ganglion. 



In these and the following stages (pi. XXVIII, figs. 35 and 36) the 

 entire nervous system and the twisting of the parietal loop may be 

 seen with diagrammatic clearness. This twisting, which is a part of 

 the general movement toward the right of all the organs posterior to 

 the foot, carries the left parietal ganglion to the right side of the foot, 

 where it lies in contact with the right pleural, while the abdominal 

 lies still farther toward the right and the dorsal side (fig. 36) ; at the 

 same time the right parietal is carried up to the dorsal midline, where 

 it lies on the dorsal side of the oesophagus (fig. 36). The entire 

 nervous system, except the buccal ganglia, Avhich are here hidden from 

 view by the cerebrals, is shown in very nearly its definitive position in 

 fig. 36. All of the six pairs of ganglia may be recognized in fig. 30, 

 and four pairs are recognizable in fig. 23. 



In all of these regards the nervous system of Fulgur shows funda- 

 mental resemblances to that of Crepidula. To one who had not ob- 

 served the earlier stages in the origin of the cerebral ganglia it might 

 seem that these ganglia, which lie so near the periphery of the blasto- 

 derm and so closely connected with the other organs of the posterior 

 lip, could not possibly have arisen from cells of the first quartet anterior 

 to the transverse arms of the cross. It is only by a study of the early 

 history of these ganglia and their subsequent movements that one can 



