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. 



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

 brj'O gradually passes into the adult form. The blastomeres 

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

 vest the larger, which form the hypoblast. 



Salensky's ^ recent observations on jB}'achionus urceolaris 

 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, w^hich is developed out of the hypoblast. 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 Jiotifera the trochal 

 disk is sometimes produced into long ciliated tentacula, 

 which surround the mouth symmetrically i^Stephanoceros^ 

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

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

 and it may be bilobed or horseshoe-shaped, as in Melicerta^ 

 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 Asplanchiia, 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 ZdUchHft fTir wiss. Zoologie^ 18T2. 



2 Huxley, Lacinutaria socialis. (Transactions of the Microscopical Society, 



8 



