ON SOMATIC SEX-CHARACTERS 69 



differences we find that they can be classified 

 according to their relation to fertilisation and re- 

 production, including the care or protection of the 

 offspring. The precise classification is of no great 

 importance, but we may distinguish the following 

 kinds to show the chief functions to which the 

 characters or organs are adapted. 



1. GENITAL DUCTS AND INTROMITTENT ORGANS. 

 According to the theory of the coelom which we owe to 

 Goodrich, in all the coelomata the coelom is primarily 

 the generative cavity, on the walls of which the 

 gametocytes are situated, and the coelomic ducts 

 are the original genital ducts. In Vertebrates we 

 find two such ducts in both sexes in the embryo, 

 originally formed apparently by the splitting of a 

 single duct. In the male one of these ducts becomes 

 connected with the testis while the other degenerates: 

 the one which degenerates in the male forms the 

 oviduct in the female, while the one which is 

 functional in the male degenerates in the female. 



Intromittent organs are formed in all sorts of 

 different ways in different animals. In Elasmo- 

 branchs (sharks and skates) they are enlarged 

 portions of the pelvic fins, and therefore paired. 

 In Lizards they are pouches of the skin at the 

 sides of the cloacal opening. In Mammals the single 

 penis is developed from the ventral wall of the 

 cloaca. In Crustacea certain appendages are used 

 for this function. There are a great many animals, 

 from jelly-fishes to fishes and frogs, in which fertilisa- 

 tion is external, and there are no intromittent organs 

 at all. 



2. ORGANS FOR CAPTURING OR HOLDING THE 

 FEMALE : for example, the thumb -pads of the frog, 



