32 



I3RY0Z0A ECTOPROCTA. 



large mantle- cavity (p). A further peculiarity is exhibited by the 

 corona, which here does not consist of long cells extending from 

 the oral to the aboral area, but is composed of numerous small 

 elements. The ectodermal furrow is hardly perceptible, but Barrois 

 (No. 9) was able to identify it. 



This larva, in its metamorphosis, follows the usual course. Here 

 also (Fig. 14, B) the first act of metamorphosis is the evagination of 

 the sucker and its transformation into the basal plate (a) of the 

 primary zooecium ; the bending over of the mantle then takes place, 

 and the fusion of its periphery with the edges of the basal plate, by 

 means of which the annular vestibular space (v) is closed in. While 



Fig. 14.— Longitudinal sections of two ontogenetic stages of a Cyclostomatous Bryozoon (after 

 Ostroumoff). A, longitudinal section through the larva of Frondipora. B, metamorphosis 

 of Ttibulipora serpens, a, adhesive plate ; /, central tissue ; p, mantle-cavity ; r, vestige of 

 the retractile disc ; s, sucker ; v, vestibulum ; x, cell-p^te (rudiment of the polypide derived 

 from the retractile disc). 



subsequently this whole structure degenerates, the polypide forms, 

 not by invagination, but by the separation of a cell-plate (./•, 

 Ostroumoff). This plate sinks in and becomes covered with 

 mesoderm derived from the central tissue. Ostroumoff's observation 

 of the appearance of a cavity lined with endothelium in the neigh- 

 bourhood of the developing nutritive tube is worthy of mention. 

 In this cavity, which degenerates later, we have perhaps a homologue 

 of the body-cavity of the fresh-water Bryozoa. 



6. Type of the Phylactolaematous Larva. 



The embryonic development and the metamorphosis of the Phylac- 

 fcolaemata has been investigated by Metschnikoff (No. 20), Nitsche 

 (No. 52), Reinhard (Nos. 54-56), Ostroumoff (No. 53), Korotneff 



