84 OLIGOCHAETA 



which runs in the axial cavity of the branchiae ; the walls are cellular and have 

 no muscular fibres. 



In Branchiura the branchiae are not lateral in position as in the last genus, but 

 dorsal and ventral. They contain a prolongation of the coelom, which, however, is 

 shut off by a diaphragm from the general coelomic cavity. The branchiae are 

 contractile owing to the presence of a layer of muscular fibres lying immediately 

 beneath the epidermis ; the latter is not ciliated. Immediately beneath the epidermis 

 is a blood-vessel on either side. The branchiae are limited to the posterior region 

 of the body. The reason for this is probably that this Tubifieid, like others, rests 

 with its head imbedded in the mud and its tail waving freely in the water. The 

 last branchiate Oligochaet is also a Tubifieid, Hesperodrilus branchiatus, in which the 

 branchiae are like those of Branchiura, but lateral instead of dorsal and ventral. 



IX. Eepeoductive System 1. 



The OHgochaeta like some other animals which are hermaphrodite possess a com- 

 plicated series of organs related to the reproductive function. We can distinguish 

 the essential organs, and those which are unessential and only concerned with 

 impregnation or the liberation of the genital products. The essential organs are of 

 course the ovaries and testes — the gonads ; these are the first part of the reproductive 

 system to make their appearance in the embryo. Then there are the ducts which 

 convey the sexual products to the exterior, the sperm-ducts and oviducts, and the 

 sperm-sacs and egg-sacs where the sexual elements undergo development ; finally we 

 have a series of organs which are concerned, in the mutual impregnation of the 

 worms ; the glands and sometimes penial setae appended to the sperm-ducts, the 

 spermathecae for the reception of the sperm during copulation, etc. The various 

 organs essential and non-essential have fixed positions in the body of the worm ; one 

 organ is found always in one segment, another in a second segment, in every species, 

 the positions being characteristic for the species or the genus or family as the case 

 may be. The table appended illustrates the varying position of the parts of the 

 reproductive system in all the families of aquatic Oligochaeta and a few earthworms. 

 The reproductive organs are segmentally arranged just as are most of the other 

 organs of the body; but, as also is the case with other organs, the metameric 

 arrangement is sometimes lost or obscured; the sperm-ducts for instance are not 

 always confined to a segment nor are they framed of a series of metamerically 



' For asexual reproduction, see under Aeolosoma and Naids. 



