36 BRYOZOA ECTOPROCTA. 



inner budding zone of the polypide is derived from a layer of embryonic tissue 

 capable of growth, lying below the ectodermal epithelium. The ectodermal 

 layer of the polypide -rudiment in the embryo also is overgrown by the 

 surrounding ectoderm. The ectoderm at this part is therefore bilaminar. The 

 inner layer represents the tissue capable of producing buds, from which the 

 inner layer of the polypide-rudiment is derived. From this tissue is developed 

 the inner layer of the first polypide-rudiment, and soon after, by its side, the 

 invagination of the second polypide, which thus, in Cristatella, is directly 

 connected with the primary polypide. 



While the polypide-rudiments are developing, we find, in Pluma- 

 tella, a zone of ectodermal cells encircling the middle of the embryo 

 and fusing with the wall of the ooecium (Fig. 17, pty This is the 

 girdle-shaped placenta already recognised by Korotneff, by means 

 of which the embryo is suspended in the uterus-sac and nourished. 

 In Cristatella this is wanting ; nourishment here takes place through 

 the ectodermal cell-plug which closes the neck of the ooecium, and 

 which is in close juxtaposition to the anterior pole of the embryo. 



In the later stages a circular fold of the body -wall develops 

 (Fig. 17 B,f), which surrounds the anterior half of the body, and 

 may be regarded as the equivalent of the mantle-fold of other 

 Bryozoan larvae. An ectodermal thickening, including the posterior 

 pole (Fig. 18 A, x), was regarded by Ostroumoff as a vestigial 

 sucker, while the anterior swollen part of the body in which the 

 polypide-rudiments appear may be considered as the equivalent of 

 the retractile disc. 



Although, in this way, the larvae of the Phylactolaemata may, without 

 undue forcing, be compared with those of the marine forms, a difficulty 

 arises from the above-mentioned circumstance that the primitive gastrula- 

 ingrowth here occurs at the anterior pole, which has been regarded as the 

 equivalent of the retractile disc. We are unable at present to overcome this 

 obstacle in the way of comparing the larvae of the Phylactolaemata with those 

 of the Gymnolaemata. 



The embryo, after developing in the manner described above > 

 becomes covered externally with cilia, and, escaping from the parent, 

 swims about freely. According to Kraepelin, it emerges through 

 the aperture of a degenerated polypide (usually that of the parent). 

 Braem, on the contrary, holds that the cavity of the ooecium 

 opens directly upon the exterior in order to allow of the passage 

 of the embryo. At this stage the larva is oval (Figs. 18 ^4 and 

 19 A), and the whole surface of its body is ciliated. At the anterior 

 pole there is an aperture leading into the large mantle-cavity, from 

 the base of which the two polypide-rudiments project. 



The attachment of the larva takes place at first by means of! 



