24 BRYOZOA ECTOPROCTA. 



in a restricted sense) is found to be distinct from the ciliated cells of the 

 ectodermal furrow. In this organ, which is evidently of glandular structure, 

 three parts can be made out The lateral parts show radially arranged cells 

 opening into tlie ectodermal furrow, while the cells of the middle part run 

 longitudinally and open into a small pit lying in front of the ciliated tuft of 

 the ectodermal furrow (Prouho). 



Further, there is in this larva a highly developed musculature, consisting of 

 lateral parietal and longitudinal muscles, and of a shell-adductor, already 

 noticed by Metschxikoff. A mesodermal cell-layer lying below the ectoderm 

 plays an important part in metamorphosis, the mesoderm-layer of the developing 

 polypide and of the primary zooecium being formed in it. 



The transformation of the larva of this type into the sedentary 

 form (primary zooid of tlie colony) takes place in the same way 

 as in other Ectoprocta ; the reader is therefore referred to the 

 description of the metamorphosis of Bugula and Lepralia, given 

 on p. 27. It need here only be mentioned that Cyplionautes, whose 

 metamorphosis has been made known through the researches of 

 Schneider and Ostroumoff, attaches itself by means of the paired, 

 disc-like organ mentioned above on the oral side of the body, and 

 on the degeneration (histolysis) of the internal organs, changes into 

 a spherical mass covered by the shell-valves, this being the rudiment 

 of the primary zooecium. While the ectocyst of the zooecium is 

 secreted on the surface of this mass, the polypide of the first 

 chamber is derived from the retractile disc through ontogenetic 

 processes to be described later (Schneider and Ostroumoff). 



With regard to the last point, the recent researches of Prouho in connection 

 with Flustrella have led to the remarkable discovery that the retractile disc, 

 like all other larval organs, becomes invaginated and undergoes degeneration. 

 During this process, a thickened disc forms out of the surrounding ectoderm, 

 mesodermal elements joining it on its inner side. This bilaminar plate, which 

 lies in the place of the degenerated retractile disc, now in its turn becomes 

 invaginated, and thus gives rise to the sac-like rudiment of the polyjiide. 

 Even though most other authors regard the polypide as derived directly from 

 the retractile disc, it must be considered probable that the ontogenetic process 

 here described in connection with Flustrella is the rule in the development 

 of the poly])ides in all Ecto2)roctous larvae. 



Pherusa tubxdosa, which, according to Pkouho (No. 28a), is also distinguished 

 by the possession of a bivalve shell, is allied to the above tyjie, and the larva 

 of Uyiwphorella, according to the same author (No. 28b), also approaches 

 this type. 



2. Type of the intestineless Chilostomatous Larva with 

 slightly- developed corona. 



The larvae of this type, in outward appearance, almost exactly 

 resemble the Alcyonidium larva. They chiefly occur in the Escharina 

 tribe, an examjDle being found in LepraUa Pallasiana, Avhich 



