IX ENTEBOPNEUSTA— GONADS 585 



out of the dorsal vessel, divides into two, one running into a tongue and the other 

 into the branchial septum next in front. The lingual vessel divides again into 

 two branches, -which are continued into the two above-mentioned vessels of the 

 lingual prongs ("tongue-bars"). 



It is not known in wliat way the blocd is again carried out of the branchial 

 tongues. 



X. The Gonads. 



The sexes are separate in the Eiiteropneusta. The gonads are 

 simple or branched sacs of various shapes which project into the 

 body cavity of the trunk, towards which, however, they are completely 

 closed. They form on each side a conspicuous longitudinal row in 

 the genital region of the trunk, which, however, is not sharply de- 

 marcated from the branchial region in front of it nor from the 

 hepatic region behind it. At the posterior end of each row of gonads, 

 a constant formation of new gonads takes place. 



The gonadial sacs open outward through simple efferent ducts 

 and genital pores, which always lie dorsally in the submedian line 

 close to, but on the outer side of, the branchial pores (Fig. 458). 



These gonads, which open laterally to the branchial pores, form 

 the row of principal gronads, and their pores are the primary 

 principal pores. 



A certain agreement in the number of the gonadial pores with 

 that of the branchial pores is sometimes found. 



The arrangement of the gonads may become complicated. 



A. One and the same gonadial sac may open outward through 

 accessory pores, which lie either medianly, or laterally, to the 

 principal pore. 



Such accessory pores are found in SchizocanUum brasiliense and 

 Glandiceps talahofi, in the latter in great numbers. 



B. Accessory gonads may occur in addition to the principal 

 gonads, opening outward through secondary genital pores. 



In Balanoglossus Kupjferi, such accessory gonads form a complete 

 row running parallel with the principal row, along its median side. 

 The same is the case in Glandiceps ialaboti, although here the accessory 

 row is not quite complete. In Balanoglossus canadensis, both principal 

 and accessory gonads occur, there being several rows of each. The 

 pores of all the gonads lie in the submedian lines, which are free 

 from muscle, and are in this case widened into broad streaks. 



AVhen accessory gonads occur in species of Pfychodera (e.g. Ft. 

 atiranfiaca, hahamensis, erythrfea) their pores always lie laterally to the 

 principal pores. 



Structure of the gonads. — The gonads consist (1) of a wall turned to the 

 cceloiiiic cavity and bchjngiiig to it, constructed of a tesselated epithelium and 

 line muscle fibres, and (2) of a massive inner germinal layer, consisting of germinal 

 cells and covering or follicle cells ; this layer is continued into the epithelium of 

 the efterent ducts. 



