vni PHYLUM MOLLUSCOIDA 355 



become detached before reaching maturity. Segmentation of the 

 ovum is complete, and a gastrula is formed by invagination. 



Certain differences in the larval history have sometimes been 

 regarded as separating very widely the Endoprocta from the 

 Ectoprocta. The former, like the latter, have a free-swimming 

 ciliated larva, provided with a corona and a ciliated disc. This 

 develops directly into the primary zooid after becoming attached 

 by means of the oral surface. The ectoproct larva also, as stated 

 previously (p. 344), becomes attached by the oral surface ; but any 

 rudiments of a zooid — such as an alimentary canal — which may 



lent 



yaruj 



iKistf 



Fig. 2S3.— Pedicellina. Showing successive stages (numbered 1 to 6) in the development of 

 zooids by budding, an. anus ; gang, ganglion ; mo. mouth ; tent, tentacles (retracted). (After 

 Hatschek.) 



have been developed, become absorbed, and the primary zooid is 

 developed at the free or aboral end of the larva, with its oral 

 surface directed upwards, away from the base of attachment. 

 The difference, however, is not so important as it may at 

 first appear, for the parts of the larval Endoproct do not remain in 

 the reversed position in which they are situated when attachment 

 first takes place, with the vestibule, mouth, and anus directed 

 downwards. Very soon a rotation is observed to take place, by 

 virtue of which the vestibule and developing tentacles, with the 

 mouth and anus, become carried to their permanent position on 

 the free-surface of the animal. 



CLASS II.-PHORONIDA. 



The position of Phoronis, a worm-like marine animal, is a 

 matter on which widely divergent views are held. On account of 

 certain strong resemblances to the Polyzoa, and, more particularly, 



A A 2 



