58 BRYOZOA ECTOPROCTA. 



obstacles to such a view have arisen from the study of the ontogenj 

 of the Phylactolaeniata. While most authors are agreed in assuming 

 that, in the Gymnolaematous larva, the retractile disc belongs to the 

 aboral region, more recent investigators are of the opinion that, in 

 the Phylactolaemata, the j^oint at which the polypide- rudiments 

 appear (and which corresponds to the retractile disc) occupies the 

 position of the vanished blastopore. We are not, at the present 

 moment, in a position satisfactorily to solve these difficulties, which 

 are due to the incompleteness of our knowledge of the ontogeny 

 of the Bryozoa, and must await further investigations. 



In the Bryozoa, as in Plioronis, metamorphosis begins with the 

 evagination of the sucker-like organ. This is followed by fixation 

 and extensive disintegration of the larval organs. In Plioronis, 

 only a few larval organs are thrown oflF, and these are replaced by 

 permanent organs {e.g., pre-oral lobe, tentacle-crown, circum-anal 

 ciliated ring), but, in the Bryozoa, the intestinal canal and the whole 

 of the body-wall of the larva undergo degeneration. The latter are 

 not simply thrown off (as are the provisional organs of Plioronis), 

 1)ut sink by invagination into the interior of the body (through 

 the formation of the vestibulum), and are there transformed into 

 the so-called brown Tjody. This degeneration affects not only the 

 larval integument, but all the provisional organs which lie in it — 

 the corona, the ectodermal furrow, and (according to Prouho) the 

 retractile disc. 



By means of these transformations, the larva reaches an attached 

 sac-like stage, and already shows on its surface the future ectoderm 

 of the i^rimary zooecium. Within it are found the remains of the 

 original central tissue and the brown body, described above, which 

 is formed of the degenerated larval organs. A primary zooecium, 

 at this stage of development, strongly recalls those individuals of 

 the Bryozoan colony in which, as is often the case, the polypide 

 degenerates. These also consist of a zooecium closed on all sides, 

 and have within them, besides strands of the funicular tissue, a 

 brown body derived through the degeneration of former polypides 



After the attachment of the Bryozoan larva, the primary polypide 

 very soon commences to form, while the disintegration of the larval 

 organs is still going on. Its rudiment is found at the upper or 

 distal pole of the primary zooecium, having been produced either, 

 as has till now been believed, by the invagination of the retractile 

 disc, or, as Prouho holds, not directly from this, but from a new 

 structure which appears at this point, as to the origin of which 



