334 EMBRYOLOGY 



system (Whitman). According to another view, however, 

 \ the two remain separate, and constitute the lateral sinuses of 

 the two sides. The other processes — the formation of the 

 septa and that of the intestinal and body musculature — 

 appear to take place in the same way as in the Cheetopoda. 

 By the growth of the mesodermal elements, the body cavity 

 may undergo a greater or less reduction. In the Rhyn- 

 chobdellidae the body cavity is still well developed, and is 

 provided with a distinct peritoneal epithelium, whereas in 

 the Gnathobdellidag it is almost entirely suppressed (Bourne). 

 It has already been mentioned that portions of the body 

 cavity are metamorphosed into parts of the blood-vascular 

 system. It has been stated that the dorsal and ventral 

 trunks of the blood-vessels take their origin from the 

 splanchnic layer, owing to a splitting of it.^ 



The Genital Organs are doubtless of mesodermal origin, 

 though the statements which are made concerning their 

 formation are little to be trusted. ^ 



The Intestinal Canal. — In both the Rhynchobdellidae and 

 the GnathobdellidsD we have already become acquainted 

 with the origin of the mesenteron from the three entoblasts. 

 These give rise to a vesicle composed of large cells, which 

 gradually resorbs the entoblasts whether enclosed within or 

 lying outside it, and becomes connected with the outer 

 world by means of an ectodermal invagination (comp. pp. 

 323 and 327). The pharynx which is formed in this way 

 presents different conditions, according as the development 

 is direct or indirect. In the first case the pharnyx, pro- 

 duced by the collaboration of entodermal, ectodermal, and 



^ [Burger (Appendix to Literature on Annelida) has made an ex- 

 tensive investigation of the formation of the body cavity, the blood- 

 vascular system, and the nephridia. He traced the establishment of the 

 coelom, its differentiation, and its relation to the circulatory system. 

 In regard to the nephridia, considerable agreement with the Oligochaeta 

 has been found. — K.] 



[However, Burger (Appendix to Literature on Annelida) has re- 

 cently given a detailed account of their origin, according to which they 

 are referable on the whole to proliferations of the peritoneal epithelium. 

 Not only the sexual glands, but also the efferent ducts, arise in this 

 way.— K.] 



