No. I.] THE EMBRYOLOGY OF CREPIDULA. iji 



and of the supraoesophageal ganglion in Mitraria suggest a 

 different view of the origin of the supraoesophageal ganglion. 

 . . . The position of the ganglion in Mitraria corresponds closely 

 with that of the auditory organ in Ctenophora ; and it is not 

 impossible that the two structures may have had a common 

 origin. If this view is correct, we may suppose that the apex 

 of the aboral lobe has become the centre of the preoral field of 

 the Pilidium and trochosphere larval forms." 



This view has been most carefully and elaborately presented 

 by Alex. Goette in his classical work EjitwickhmgsgescMchte der 

 Warmer. He divides bilateral animals into two groups : (i) 

 the pleurogastric, in which the chief axis of the egg becomes 

 the chief axis of the larva or adult, e.g., Sagitta and the echino- 

 derms, and (2) the hypogastric, in which group one of the 

 " cross axes " of the egg becomes the chief axis of the larva or 

 adult, e.g., worms, mollusks, arthropods. Goette has striven to 

 show that in all bilateralia the animal pole {Sckeitelpol) corre- 

 sponds to the future cephalic pole (Hirnfeld), and so far as his 

 hypogastric forms are concerned his views on this subject have 

 found repeated and abundant confirmation in the more minute 

 and exact studies which have been made during the last few 

 years on the development of annelids and mollusks. 



3. Beginnings of Final Asymmetry . 



The development of the characteristic asymmetry of the 

 gasteropod belongs to a later period than is treated of in this 

 paper. However, the beginnings of that asymmetry are clearly 

 marked during the embryonic period, and may be briefly touched 

 upon here. 



The first evidence of asymmetry and the first trace of antero- 

 posterior elongation appear at the same time and are apparently 

 due to the same cause, viz., the formation of the fifth quartette. 

 The posterior members of this quartette, 5C and 5D, are cut 

 off from the posterior side of the macromeres C and D (Figs. 

 57-60), thereby increasing the length of the median axis, and, 

 because 5C is formed earlier and lies nearer the dorsal side than 

 5D, a certain torsion of the posterior end of the embryo follows. 

 This torsion increases until 5D lies nearly on the mid-ventral 



