ABDOMINAL PORES 



401 



a single main duct opening into the terminal part of the meso- 

 nephric diict, and these tubules and their separate duct are 

 sometimes regarded as a metanephros and a metanephric duct. 

 The mesonephric nephrostomes are persistent throughout life in 

 a few Elasmobranchs (e.g. Notidanidae, Heterodontidae, Ehinidae, 

 and some ScylUidae), and also in Amia : ^ in all other Fishes 

 as well as in the Cyclostomes they become closed in early 

 Ufa. 



In many Fishes the hinder extremity of the coelom com- 

 municates directly with the exterior through " abdominal pores," 

 of which there is usually a 



pair, rarely a single pore, 

 situated close to the cloacal 

 or the anal aperture.^ Elasmo- 

 branchs usually have a pair, 

 often at the extremities of a 

 pair of cloacal papillae (Fig. 

 231), but they are absent in 

 some families (e.g. Hetero- 

 dontidae and Ehinidae) ; and 

 in some Scylliidae (e.g. Scyl- 

 lium cariicula) they are very 

 variable, being either present 

 or absent on both sides, or an 

 open pore is present ou one fig. 231 

 side only. Pores are present 

 and paired in the Crosso- 

 pterygii, the Chondrostei, and 

 the Holostei. Amongst the 

 Dipnoi Neoceratodus has a pair 

 of pores. Proto'jpterus some- 

 times has two pores opening 



Diagraniniatio horizontal section 

 through the abdominal pores and cloaca 

 of an Elasraobranch. a.p. Abdominal 

 pore ; c, coelom ; d, cloaca ; c?.^j, cloacal 

 papilla ; c.p, cloacal pit ; od, oviducal 

 apertures in the female ; r, rectum ; 

 «.,?, cloacal aperture of the urinary sinus 

 (female), or the urogenital sinus (male). 

 In some Elasmobranchs the abdominal 

 pore opens at tlie base of the cloacal 

 papilla, as shown at a.p^. (Modified from 

 Bks.) 



into the cloaca, but as a rule 

 the two become confluent and have a single external aperture. 

 In Zepidosiren pores are wanting. Abdominal pores are rarely 

 present in Teleostei. They exist, however, in the Mormyridae 

 {Gymnarchus and several species of Mormyrus), and also in the 



' Jungersen, Zool. Am. xxiii. 1900, p. 328. 



- Bridge, Journ. Anat. and Phys. xiv. 1879, p. 81 ; Bles, ib. xxxii. 1898, p. 

 484 ; Proc. Soy. Soc. Ixii. 1898, p. 232. 



VOL. VII 2 D 



