170 E. B. Cumings —Development of Fenestella. 



colonies, the zooecia emerge upon the surface ; but in the older 

 ones, the apertures of the zooecia in the basal portion are sub- 

 merged in a copious deposit of punctate sclerenchyma. In all 

 cases, however, there has been no resorption of the earlier 

 zooecia, so that sections of the bases of ephebastic or gerontas- 

 tic zoaria reveal the morphology of the earliest stages as 

 faithfully as sections of a nepiastic colony. As an aid to the 

 elucidation of the astogeny of Fenestella, the writer studied 

 the astogeny of Retepora phoenicea, a recent bryozoan morpho- 

 logically very similar to the ancient Fenestellas and Poly- 

 poras. 



In the writer's former paper on the development of Paleo- 

 zoic Bryozoa, the term protcecium was introduced as designating 

 the primary individual of the colony. In this sense, it w^ould 

 have the same signification as the term ancestrula of Jul lien 

 or primary cell of Hincks. In the Cyclostomata, as is well 

 known, the first zooecium surmounts a hemispherical base 

 (basal disc), which serves as the point of attachment of the 

 young colony to the substratum. This basal disc has been 

 shown to be the calcified wall of the metamorphosed and 

 histolyzed embryo (Barrois and others). It is believed by the 

 present writer that the persistence of this structure (kathem- 

 bryonic stage) in the ancient order of Cyclostomata is not 

 without significance, especially in view of the fact, to be 

 shown presently, that it is a conspicuous feature in the 

 development of the ancient Cryptostomata and possibly of the 

 Trepostomata (Phylloporina corticosa). The basal disc is 

 probably the true first zooecium. In the present paper, there- 

 fore, the term protcecium is restricted to the basal disc or its 

 equivalent, and the superjacent portion of the primary cell is 

 designated the ancestnda. In many recent Chilostomata, 

 there seems to be no distinction of protcecium and ancestrula. 

 This may mean that the extreme acceleration of these modern 

 types has practically eliminated the protcecium from the on- 

 togeny, hi the ancient Cryptostomata, on the other hand, the 

 protcecium greatly predominates over the ancestrula, which is 

 often little more than an exaggerated aperture to the former. 

 In any case, the ontogenetic stage of which the protcecium is 

 the index is always present throughout the Ectoprocta, for by 

 a degenerative metamorphosis they all give rise to a hemi- 

 spherical kathembryo, from which the adult polypide arises by 

 a sort of budding process. Furthermore, this kathembryo 

 becomes invested with a calcareous or chitinous ectocyst, which 

 is the first skeletal structure of the developing individual. 

 The protcecium is therefore very closely analogous to the 

 protegulum of brachiopods, the protoconch of cephalopods, 

 etc. 



