RESEARCH SEMINAR. 321 



but later disappear. The vessels of the mandibular arch are only 

 slightly indicated. 



The veins are arranged symmetrically during the early stages. 

 The arrangement of the cardinal veins is typical. A well-marked 

 subintestinal vessel is present very early ; later it disappears in 

 part, and in part becomes connected with the hepatic-portal vein. 

 The vena cava is for a time independent of the cardinal system ; 

 later the anterior part of the right posterior cardinal disappears, 

 while the vena cava connects with its posterior section. The 

 pulmonary vein appears very late. 



Many of the features of the development of the vascular sys- 

 tem are closely similar to those of the Urodela. Some, though 

 not all, of the elasmobranch resemblances appear quite late in the 

 process of development and are preceded by conditions which are 

 typically amphibian. There are, however, typical elasmobranch 

 characters present from the first. 



August 12. The Metamerism of the Nervous System in Areni- 



cola cristata. By C. P. LOMMEN. 



The arrangement of the nervous system of Arenicola cristata 

 is definitely related to the annuli forming its somites. A pair of 

 nerves from the ventral cord proceed dorsally between the longi- 

 tudinal and circular muscles along each of the grooves that sepa- 

 rate the annuli. These nerves give off numerous tiny branches, 

 and do not unite dorsally to form rings. In the setigerous annuli 

 there are two additional nerves which are imbedded in the circu- 

 lar muscles, one on each side of the neuropodium. The posterior 

 one divides into two branches, one passing into the gill and the 

 other into the seta-sac. Anteriorly the number of annuli in the 

 somites is gradually reduced from five to two. From the outside 

 of each connective a series of eight nerves is given off, all of 

 which are presumably homologous to the nerves from the cord. 

 This homology has been shown with certainty only in the case 

 of three. Of the remaining five, one innervates the otocyst, the 

 position and muscular connection of which are suggestive of 

 homology with a seta-sac. Fifteen nerves from the inside of the 

 connectives give off some branches to the body-wall and then 

 bend back into the wall of the pharynx, innervating its ventral 



