312 EMBRYOLOGY 



the anus is formed at the posterior end of the ventral side. The anal 

 vesicles are said by Spengel to arise as evaginations of the hind-gut 

 (Fig. 148 C). We saw that in Echiurus their origin was described 

 differently, and that therefore they are pi'eferably to be considered as 

 nephridia (p. 309).— A pair of tubes which make their appearance behind 

 the mouth are considered by Spengel to be the provisional excretory 

 organs (Fig. 148 C). Immediately behind these the ventral setae are 

 formed (Fig. 148 C). 



The earliest fundament of the ovary was also observed by Spengel. 

 It is formed, in the same way as in other Annelids, from the peritoneal 

 covering of the blood-vessels, in this case on the posterior part of the 

 ventral vessel. The duct for the sexual products is a tube, which is to 

 be looked upon as a nephridium, although it is not quite evident whether 

 it is connected, and if so by what means, with the provisional excretory 

 organs observed by Spengel. 



The description of the development of Bonellia np to this point 

 applies to the female only. The development of the small male, living 

 in the uterus, is much simpler, since it remains in the state of the 

 ciliated larva. The larvae which develop into males seek the ciliated 

 groove on the prostomium of the female, and there attach themselves. 

 They lose the two ciliated bands, but retain the uniform coat of cilia. 

 Their internal organization corresponds on the whole to that of the 

 female, only certain simplifications arise ; thus, for example, the mouth 

 and anus are wanting. In the male also the genital products arise from 

 the cells of the peritoneum. Balls of spermatic cells are detached from 

 this and fall into the body cavity, subsequently to be taken up by the 

 funnels of the spermatic duct. After the males have remained for a 

 short time on the prostomium of the female, they migrate into the 

 oesophagus, in order to complete there their metamorphosis. Spengel 

 found as many as eighteen males in the ajsophagus. Subsequently they 

 abandon the oesophagus and repair to the uterus, where ordinarily six, 

 eight, or more males are found. 



General Considerations. — As regards the position of 

 the Echiurida?, we agree with Hatschek's view (No. 51) ; he 

 sees in them a division of the Annelida, and brings them 

 into relation with the Chcetopoda. The form and internal 

 organization of the larva, as well as the mode of origin of 

 the setae, seem fullj to substantiate this view. Even though 

 a segmentation [metamerism] no longer exists in the adult 

 animal, it was nevertheless established in the larva, just 

 as in the Chietopoda and Archiannelida. The loss of the 

 segmentation and the reduction of the setae, as well as the 

 enormous extension of the prostomium, or so-called proboscis, 

 make the EcMurldie appear as somewhat modified forms. 



