EMBRYONIC FISSION IN OYCLOSTOMATOUS POLTZOA. 9 



" mantle-cavity." I have observed no trace of a " pyriform 

 organ." Barrois (2, p. 142; 3, p. 43, pi. iii, fig. 31) states, 

 however, that he has discovered a rudiment of this structure 

 in the larvse of Discopora. 



The internal structure of a young larva may be illustrated 

 by means of fig. 23, a median longitudinal section. The 

 sucker is already well developed, having been formed, as in 

 most other Ectoproct larvae, by a process of ectodermic in- 

 vagination. Cilia have appeared on the greater part of the 

 external surface, the non-ciliated part of the ectoderm repre- 

 senting the portion which will be later invaginated to form the 

 mantle cavity. The inner layer of cells is still perfectly dis- 

 tinguishable, forming a thin layer, closely applied to the ecto- 

 derm, and enclosing a cavity which occupies the whole of the 

 interior of the embryo. 



The earlier stages, which alone concern us at present, may 

 be realised by assuming that the volume of the embryo shown 

 in fig. 23 has become largely reduced ; and that the sucker 

 has become flattened out. Slightly anterior to the stage of 

 fig. 22, the sucker is much shallower, and opens by a wide 

 aperture in the middle of the " oral " surface. Still earlier, 

 the sucker is a very slight depression of the thickened "oral " 

 ectoderm. The inner layer is at this stage a layer of great 

 tenuity, in which a nucleus is thick enough to form a 

 swelling wherever it occurs. Before this, the embryo is plano- 

 convex, the position of the future sucker being represented by 

 its flat side; and, still earlier, it is rounded in section, the 

 inner layer consisting of a few cells, completely surrounding a 

 central cavity. Between this stage and that shown in fig. 22, 

 the inner layer may be separated, in parts or completely, from 

 the ectoderm ; so that it would be impossible to overlook its 

 presence in any well-preserved section. 



At the earliest stage at which the embryo is free in the ovi- 

 cell, it consists of a small rounded mass (PI. II, fig. 11) 

 The outer layer is in the form of a continuous mass of proto- 

 plasm, enclosing one layer of nuclei. The inner layer also 

 consists of continuous protoplasm, with a very small number 



