44 GEPHYREA. 



GEPHYREA. 



(40) Keferstein u. Ehlers. Zoologische Beitrage. Leipzig, 1861. 



(41) C. Semper. Holothurien, 1868, p. 145. 



(42) J. W. Spengel. " Beitrage z. Kenntniss d. Gephyreen." Beitrage a. d. 

 zool. Station . Neapel, Vol. i. 1879. 



(43) J. W. Spengel. "Anatomische Mittheilungen lib. Gephyreen." Tagebl. 

 d. Naturf. Vers. MUnchen, 1877. 



In the Gephyrea, as in the Chsetopoda, the ova are developed 

 from the lining cells of the peritoneum and frequently from the 

 cells surrounding parts of the vascular system (Bonellia, Thalas- 

 sema). In many cases (Sipunculus, Phascolosoma, Echiurus) 

 the main growth of the ovum takes place after it has been 

 dehisced into the body cavity. 



In Sipunculus the ova in the body cavity are surrounded by- 

 a follicle which is thrown off before they become ripe. 



Brandt denies the existence of this follicle or rather its cellular nature 

 Spengel's (43) observations are conclusive in favour of the correctness of 

 the original interpretation of Keferstein and Ehlers. The follicles would 

 seem to be formed after the ova have become free. In Phascolosoma there 

 is no follicle (Semper, Spengel). 



In both Phascolosoma and Sipunculus a vitelline membrane 

 with radial pores zona radiata is formed, and in Phascolosoma 

 the external part of this is separated off as a structureless 

 vitelline membrane. The formation of both these membranes 

 from the protoplasm of the ovum is rendered certain in the 

 latter case by the absence of a follicular epithelium. 



Some interesting observations on the growth and origin of 

 the ovum in Bonellia have been made by Spengel. 



The ova originate from certain cells (germinal cells) in the 

 peritoneal investment of the ventral vessel, overlying the nervous 

 cord. These cells, which are well marked off from the surround- 

 ing flattened peritoneal elements, increase in number by division, 

 and form small masses surrounded by a follicle of peritoneal 

 cells, and attached by a stalk to the peritoneum. The central 

 cell of each mass grows larger than the rest, which arrange 

 themselves in a columnar fashion round it ; it is not, however, 

 destined to become the ovum. On the contrary certain of the 

 other cells adjoining the stalk grow larger, and finally one of 

 these becomes distinguished as the ovum by its greater size and 



