616 A. M. CAKli SAUNDKKS AND MARGAUET POOLE. 



breaks throu<jh and comes into communication with the seg- 

 mentation cavity. When this occurs, the anterior wall of the 

 latter is still in part formed by the two endomeres, A and jB. 

 This, however, soon ceases to be the case. At this stage 

 the intestine grows out as a tube-like evagination from the 

 right posterior portion of the stomach, and reaches the surface 

 just behind the anal cells. The anus is formed at once, and 

 there is but a very slight ectodermal invagination, forming 

 only the lip of the aperture. In this respect Aplysia agrees 

 exactly with Umbrella. In this form the intestine arises 

 from the derivatives of 5c and od ; and this is probably the 

 case also in Aplysia. 



The oesophagus is long and narrow, and almost entirely 

 ectodermal. From an early period it is ciliated, in the fi-ee- 

 swimming larva the cilia being very long and numerous, 

 and in sections tilling up almost the whole lumen of the 

 tube. Mazzarelli describes a cuticle which lines the cavity 

 of the oesophagus of Opisthobranch larvie ; our preparations 

 of Aplysia certainly do not show this structure. 



The stomach wall is formed by rather small, clearly defined 

 ciliated cells, constituting a columnar epithelium. In the 

 embryonic stages the cilia are all alike throughout the lining 

 of the cavity, but in the free-swimming larva, in the posterior 

 region they are replaced by stiff hair-like structures, which 

 Mazzarelli calls " bastoncelli." They are probably fused 

 cilia, and serve as a staining apparatus. The fact that they 

 are not developed until the larva becomes free-swimming 

 supports this view, for until that stage is reached the embryo 

 feeds upon the yolk stoi-ed in the liver, and does not take in 

 food through the mouth. Xo epithelium can be seen covering 

 the wall of the stomach externall^^ The intestine is a simple 

 ciliated tube, and also appears to lack an epithelial investment. 

 At first (PI. 22, fig. 4) it arose from the right ventral poste- 

 lior region of the stomach, but in the course of the torsion, 

 which affects the whole of this part of the body, it becomes 

 c.irried up on to the right side (PI. 22, figs. 6 and 7) ; and 

 finally, in the free-swimming larva it is seen to pass from the 



