ON THE LTFE-HISTORY OF PEDIOELLINA. 155 



at least of the portions described in the last stage, has become 

 somewhat simplified. All the more ventral regions (situ- 

 ated in the neighbourhood of the surface of attachment) have 

 completely disappeared, and in their place is found a mass of 

 cells filling a cylindrical stalk, which obviously corresponds to 

 that of the adult Pedicellina. The anal division of the ves- 

 tibule has continued its backward growth and now lies almost 

 at the free end of the young animal. At about this stage it 

 acquires a secondary opening to the exterior on the side corre- 

 sponding to the posterior surface of the larva. This opening 

 is formed by a simple concrescence between the vestibular 

 epithelium and the external ectoderm of the body, accompa- 

 nied by a linear perforation formed at the point of junction 

 of these two distinct portions of ectoderm. My sections have 

 given me no indication of the occurrence of a " labial invagi- 

 nation" (Barrois, q. v.) placing the above portion of the ves- 

 tibule in connection with the exterior. 



The character of the vestibular aperture, immediately after 

 its formation, may be seen from fig. 1], a section passing in a 

 plane corresponding to gh in fig. 10. The vestibular aperture, 

 at the sides of which tentacles (t.) are already developing, is 

 shown, by an examination of the remaining sections of the 

 series, to have the form of a slit elongated in the direction of 

 the median plane of the animal. Immediately before the for- 

 mation of the aperture the vestibular epithelium would appear, 

 in a section of this kind, quite unconnected with the external 

 ectoderm, but already extending towards it in the form of a 

 median groove, similar in appearance to the portion g, v. in 

 fig. 11. 



The mouth in fig. 10 has, at first sight, the appearance of 

 being closed. By a comparison, however, of fig. 10 with 

 fig. 16, it would seem that the apex of the epistome is really 

 represented (in the former) by the ectoderm closing the (per- 

 manent) mouth, and it is thus probable that the commence- 

 ment of the digestive tube in fig. 10 (v. or.) is a part of the 

 oral division of the vestibule. This impression is strongly 

 confirmed by a section (not figured) similar to, but later than. 



