GEPHYREA. 299 



become mature, though the seminal pouch of the adult is not found 

 in many of the males living in the oesophagus. When mature the 

 males leave the oesophagus, and pass into the uterus. 



Phascolosoma. Cilia appear in Phascolosoma (Selenka. No. 369) 

 while the ovum is still segmenting. After segmentation they form 

 a definite band immediately behind the mouth, which divides the 

 larva into two hemispheres a prte-oral and a post-oral. A prae-oral 

 band of cilia is. soon formed close to the post-oral band, and at the 

 apex of the prae-oral lobe a tuft of cilia also appears. 



The larva has now the characters of a trochosphere, but differs 

 from the typical trochosphere in the post-oral part of the ciliated 

 equatorial ring being more important than the prse-oral, and in the 

 absence of an anus. 



The metamorphosis commences very early. The trunk rapidly 

 elongates, and the praa-oral lobe becomes relatively less and less con- 

 spicuous. The zona radiata becomes the larval cuticle. 



Three pairs of bristles are formed on the trunk, of which the 

 posterior pair appears first, then the anterior, and finally the middle 

 pair : an order of succession which clearly proves they can have no 

 connection with a true segmentation. 



The tentacles become developed between the two parts of the 

 ciliated rings, and finally the prae-oral lobe, unlike what takes place in 

 the Gephyrea armata, nearly completely vanishes. 



The anus appears fairly late on the dorsal surface, and the 

 ventral nerve cord is established as an unganglionated thickening of 

 the ventral epiblast. 



GEPHYREA TUBICOLA. 



The larva of Phoronis was known as Actinotrocha long before 

 its connection with Phoronis was established by Kovvalevsky (No. 

 372). There is a complete segmentation leading to the formation 

 of a blastosphere, which is followed by an invagination, the opening 

 of which is said by Kowalevsky to remain as the mouth 1 . It is at 

 first terminal, but on the development of a large prae-oral lobe it 

 assumes a ventral position. The anus is formed at a later period at 

 the posterior end of the body. 



The youngest free larva observed by Metschnikoff (No. 373) was 

 less developed than the oldest larva found by Kowalevsky. It pro- 

 bably belongs to a different species. The body is uniformly ciliated 

 (fig. 163 A). There is a large contractile prse-oral lobe, and the body 

 ends behind in two processes. The mouth (ni) is ventral, and the 

 anus (an) dorsal, and not terminal as in Kowalevsky 's larva. 



The alimentary tract is divided into stomodseum, stomach and 

 intestine. The two processes at the hind end of the body are 

 the rudiments of the first-formed pair of the arms which are so 



1 Kowalevsky states that what I have called the mouth is the anus, but his sub- 

 sequent descriptions shew that he has transposed the mouth and anus in the embryo, 

 aiid that the opening, which he asserts to be the anus, is in reality the mouth. 



