390 



COMPARATIVE ANATOMY 



described as genital pores, comparable to the like-named pores of 

 Teleosts (cf. under Generative Organs). 



Apart from the indirect connection of the coelome with the 

 exterior by means of the oviducts in the female, such as exists 

 in most Vertebrates, other connections are seen, for example, in 

 both male and female Elasmobranchs. These are either indirect, 

 through the nephostomes of the kidney (q.v.) t or direct, through 

 the abdominal pores : in some cases, both these means of communi- 

 cation with the exterior exist in the same Elasmobranch, but this 

 is never the case in other Anamnia, so "that they appear to be to a 

 great extent mutually exclusive. 



In the Elasmobranchii the abdominal pores are usually paired 

 and are situated posteriorly to the cloaca (Figs. 296 and 297), and 



---* b, b f 



FIG. 296. DIAGRAMMATIC HORIZONTAL SECTION THROUGH THE CLOACAL 

 REGION OF A PLAGIOSTOME. (After E. J. Bles.) 



a, blind ectodermal invagination (cloacal pouch) ; ft, ft 1 , cloaca! papilla ; c, peri- 

 toneal cavity, which opens by the abdominal pore at c 1 ; Cloake, cloaca ; 

 Darm, rectum ; ***, points along the transversely striped section of the 

 cloacal papilla at which the abdominal pore may break through, in which 

 case the distal part of the papilla is solid (Raja). 



may be enclosed within its lips. They are wanting in the Noti- 

 danidge, Cestracionidse, and Rhinida3, and are not constantly 

 present in the Scylliidae and others, even in individuals of the 

 same species, and they may only appear at sexual maturity. In 

 Ganoids, they open between the urinogenital aperture and' anus, 

 but are apparently wanting in Arnia when sexually mature. 

 Amongst Teleosts, they are said to be present only in the 

 Salmonidae and Monnyridae, right and left of the anus ; but even 

 in these, the pore of one or of both sides may be absent. In 

 Ceratodus the abdominal pores are paired, and open behind the 



