ON THE LTPB-HISTOEY OP i'EDIOELLINA. 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 g h 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. 



