TROCHOPHORE OF HYDROIDES UNCIXATUS (ErpOMATUS). 585 



small, closes from behind forwards, the anterior portion 

 remaining as the mouth, while the posterior closes completely, 

 the anus breaking through immediately at this point. The 

 blastopore, which was originally ventral, becomes shifted to 

 a ventro-lateral position. At a time when gastrulation is 

 about half completed, some cells appear on either side of the 

 eiidoderm and take up a bilateral position. They probably 

 correspond to the lateral portions of the larval or ectomesoblast 

 of Thalassema. They subsequently form the head-kidneys 

 in Eupomatus. At the same time two conspicuous cells are 

 usually distinguishable in the ventral lip of the blastopore. 

 These are the coelomesoblast cells. In the further progress of 

 invagination, they are carried inwards in the wall of the arch- 

 enteron, finally coming to lie in the anal end of the gut. Here 

 at a considerably later stage they give rise to the mesoderm 

 bands. There is a short stage in the early trochophore when 

 the head-kidneys are already functional while the mesoderm 

 bands are alone represented by these two cells in the gut- 

 wall. With the formation of the bands the organisation of 

 the trochophore is completed. The bauds during their 

 growth are never seen to bud off cells into the blastocoel. 

 They remain from the first a compact mass of cells clearly 

 distinguishable from the irregular cells of the ectomesoblast 

 and the head-kidneys. The head-kidneys oj^en into the 

 proctoda3um. They are formed from the ectomesoblast. 



LiTEKATUKE. 



1. Agassiz, A. — " On the Young Stages of a Few Annelids." • Annals 



Lyceum Nat. Hist. New York,' vol. viii, 1867. 



2. Andrews, E. A. — " Filose Activity in Metazoau Eggs," " Zool. 



Bullet.,' vol. ii, 1898, p. 1. 



3. Bergli. R. S. — " Zur Entwicklung iind Differenzirimg des Keims- 



triefens von Lumbricus,"' ' Zeit. f. wiss. Zool.,' vol. 1, 1890, 

 p. 469. 



4. Bloclnnann, F. — " Ueber die Entwicklung der Neritiuu fluvia- 



tilis." ' Zeit. f. wiss. Zool.,' vol. xxxviii, 1883. p. 392. 



