THE ROTIFERA. 169 



lieving that the summer ova are occasionally, if not alwaj^s, 

 developed without fecundation, and that it is the winter ova 

 which are fecundated. 



Tlie egg- undergoes complete yelk-division, and the em- 

 bryo gradually passes into the adult form. The blast omeres 

 are soon of unequal sizes, and the smaller, as an epiblast, in- 

 vest the larger, which form the hypoblast. 



Salensky's ^ recent observations on JBrachio7ius iirceolaris 

 show that a depression arises on one face of the epiblast and 

 that the antero-lateral parts of this depression are converted 

 into the trochal disk, while its median posterior part grows 

 out into the "foot; " and he points out the resemblance of 

 the embryo in its early stages to that of some Gasteropods. 



An involution of the epiblast at the bottom of the depres- 

 sion gives rise not only to the oral chamber, but also to the 

 mastax ; eventually communicating with the gastro-intestinal 

 division, which is developed out of the hyj)oblast. The gan- 

 glion is a product of the epiblast. 



Some of the modifications of the general structure thus 

 described, which occur in the different groups of the Hotife- 

 ra, are of considerable interest. 



Thus, in the tubicolous forms, the body is elongated and 

 terminated posteriorly by a discoidal surface of adhesion. 

 The animals (of which a number are often associated together), 

 fixed by this disk, inclose themselves in cases, the foundation 

 of which is a gelatinous secretion. The intestine is bent 

 upon itself {Lacinularia, Fig. 39, II.), and opens upon the 

 face of the body opposite to that upon which the ganglion is 

 placed. The peduncle of attachment is therefore a process of 

 the neural face of the body. In these Rot if era the trochal 

 disk is sometimes produced into long ciliated tentacula, 

 which surround the mouth symmetrically (Stephanoeeros, 

 Ficr. 39, v.), or its edges may be provided with two circlets of 

 cilia, one in front of, and the other behind, the oral anerture ; 

 and it may be bilobed or horseshoe-shaped, as in 3Ielicerta, 

 and Lacinularia ^ (Fig. 39, I., II.). 



In the free Rotifers, the body may be rounded, sac-like, 

 and devoid of appendages, as in the genus AsplancJma^ which 

 has neither anus nor intestine. In Albertia and Lindia^ on 

 the other hand, the body is elongated and vermiform. Most 

 of the free JRotifera (Fig. 38) are provided with a segmented 



1 ZnUclirift fi'ir wiss. Zoohffie^ 1872. 



2 Huxley, Lacinularia socialls. (Transactions of the Microscopical Society, 

 1851.) 



