240 SUMMARY OF CURKENT RESEARCHES RELATING TO 



closely resembles that (if Lichnmolgit.<>. When freed from the gill- 

 tubes the females swim about with consideraljlo velocity considering 

 their jirevious imprisonment. It is doubtful by what channel they 

 reach their resting-place. 



Vermes. 



Formation of Trochosphere in Serpula.* — H. W. Conn describes 

 the development of Scrpnia and founds on it and that of Thalassema 

 a new theory as to the relations of larvfe. 



In Serpida the segmentation is regular and complete, as Stossich 

 has stated ; but the history of the blastopore is very different from 

 that described by him. The regular segmentation is followed by a 

 typical invagination, giving rise to a gastrula. A band of cilia now 

 makes its appearance around the blastopore and a tuft of sensory cilia 

 at the opposite end. Now that part of the body within the circum- 

 blastoporal ciliated ring, and therefore containing the blastopore, 

 begins to elongate obliquely, causing an elongation of the blastopore. 

 The axis of this elongation does not fall through the centre of the 

 blastopore, but through one edge of it, and the elongation is therefore 

 such that one end of the blastopore remains near the ring of cilia 

 while the other is carried away at the end of the elongated portion of 

 the body. As this elongation progresses the blastopore, which has 

 become drawn out into a long slit, closes, and its lips fusing together 

 become what proves to be the ventral median line of the full-grown 

 larva. The endoderm is entirely within the body-cavity, connected 

 with the ectoderm, however, throughout the whole extent of the 

 closed blastopore. Soon the two extremities of this closed blasto- 

 pore open again, the one near the ciliated band becoming the 

 mouth, the other eventually becoming the anus, while the endo- 

 derm between these two points loses all connection with the ecto- 

 derm, except at the mouth and anus, and becomes hollowed out to 

 form the alimentary canal. The blastopore is not therefore converted 

 into the anus as Stossich asserts, but it elongates, one extremity 

 eventually becoming the mouth, the other the anus, while the inter- 

 mediate portion closes to form the median ventral line of the larva 

 and full-grown adult, 



Mr. Conn's theory is in brief as follows : — As a simplest type of 

 larva and the most universal is found a form which agrees in essential 

 respects with the young pilidium of Nemertians. It consists of just 

 such a gastrula as in Serptda. This type is present in Coelenterata, 

 Polyzoa, Brachiopoda, Vermes, and Molluscs, and, in a slightly modi- 

 fied form, i. e. without the circumblastoporal ring, in Echinoderms. 

 Omitting the latter, the above groups are divided into two radically 

 distinct classes. In the first class (Coelenterata, Polyzoa, and Brachio- 

 poda), the body of the larva and the adult is formed by the elongation 

 of that part of the gastrula body situated in front of the cii'cum- 

 blastoporal ring, between it and the anterior ciliated tuft. In the 

 second class (Molluscs, Annelids, and probably other worms), the body 



* JohiiB-Hopkins Uuiv. Circulars, iv. (1884) pp. 1.")-1G. 



