328 COMPARATIVE ANATOMY. 



connection with this apparatus which looks like a series of 

 surgical instruments, there is a gland, surrounded by muscular 

 fibres, which is formed as an involution of the integument ; in 

 its histological character this calls to mind the uropygial gland 

 of Birds. 



Amongst the Amphibia, male Gymnophiona alone possess 

 a true copulatory organ; this simply consists of the eversible 

 cloaca, which reaches a length of five centimetres, and is regu- 

 lated by a well-developed musculature. In Urodeles there 

 is merely a marked swelling of the lips of the cloaca and urino- 

 genital papilla during the breeding-season. 



Two kinds of copulatory organs are found in Reptiles, the 

 one being seen in Lizards and Snakes, and the other in Chelonians 

 and Crocodiles. 



In the former case, there are two erectile penes lying outside 

 the cloaca, under the skin at the root of the tail. By a compli- 

 cated muscular mechanism these can be drawn into the cloaca and 

 thence everted, the seminal fluid passing along a spiral furrow in 

 each. Similar organs are present though much less developed 

 in the female also. 



Chelonians and Crocodiles possess a copulatory organ which is 

 united to -the ventral wall of the cloaca : it consists of two fibrous 

 masses which are fused together in the middle line. Each half 

 encloses a large cavity, which contains a large number of blood- 

 vessels. The organ is regulated by well- developed muscles, and 

 possesses a groove along its free surface, which may divide up 

 into a series of channels at its apex. A representative of this 

 organ is also present in the female. 



In most Ratita3, as well as in some Carinatse (e.g. in Water- 

 Birds), a copulatory organ is present, and consists of an eversible 

 tube, strengthened by two fibrous bodies ; in a condition of rest it 

 lies coiled up in the left side of the cloaca. The everted organ is 

 retracted by means of an elastic band. The penis of the Ostrich 

 lies within a diverticulum of the cloaca, and somewhat resembles 

 that of Chelonians and Crocodiles. 



The copulatory organs of Mammals may be divided into two 

 groups, viz., those of Monotremes, and those of other Mammals : 

 the organs of Marsupials may be placed in a subdivision of the 

 latter. In all cases the female apparatus (clitoris), although as 

 a rule less developed and usually not perforated by the urethral 

 canal, is formed essentially on the same type as that of the male. 

 In Monotremes, the copulatory organ lies enclosed within a 

 sac situated on the boundary between the urinogenital sinus and 

 cloaca, and is fused with the ventral wall of the latter (Fig. 251, 

 Gp) : in all other Mammals the organ arises in the embryo from 

 the "genital prominence" on the ventral wall of the cloaca. 

 A channel passes along the side facing the cloaca to the opening of 

 the urinogenital sinus : this condition is usually retained through- 



