360 



OLIGOCHAETA 



the altered dorsal glandular pouch already spoken of, which is 

 surrounded by a blood sinus. 



Reproductive Organs. All the Oligochaeta are hermaphro- 

 dite animals. But, as is the case with other hermaphrodites, the 

 male and female organs are in many cases mature at different times, 

 thus leading to a practical unisexuality. Many of the aquatic 

 forms appear to have fixed times for breeding, which may be in 

 the winter or in the summer; but the earthworms are as a rule 

 .sexually mature the whole year round. Various accessory organs 

 are developed in the majority of cases. In all, the reproductive 

 glands lie in successive segments and are attached to the septa, 

 from the peritoneal covering of which they originate. Their 

 actual position differs greatly in different genera ; the position is 

 constant only in the earthworms, where the testes are in the 

 tenth and eleventh segments and the ovaries in the thirteenth, in 

 exactly corresponding situations. A few earthworms have only 

 one pair of testes. The only exception, among terrestrial forms, 

 to the position of the generative organs is in the family Moni- 

 ligastridae, which show so many other affinities to the lower 

 forms of Oligochaeta. In this family the ovaries have moved 

 one or two segments forward. Among the fresh-water families 

 the position of the testes and of the ovaries is not so uniform. 

 They are generally more anterior than in the terrestrial genera, 

 particularly the ovaries. 



One of the chief differences between the Oligochaeta and the 

 Polychaeta is that the reproductive organs of the former have 

 special ducts to convey their products to the exterior. In Aeolo- 

 soma, the only exception to this rule, Dr. Stole * has shown some 

 reasons for believing that certain nephridia, but slightly altered 

 in form, serve as the conduits of the spermatozoa, whilst the 

 ova are extruded through a pore upon the ventral surface of 

 the body. In the Enchytraeidae the same pore for the extru- 

 sion of the ova appears to exist ; but a nearer examination 

 shows that it is really not a mere perforation of the integument, 

 like the dorsal pores, for example, but that its internal orifice is 

 fringed with cells which seem to represent a rudimentary oviduct ; 

 perhaps Aeolosoma typifies a last stage in the reduction. Even 

 so high in the scale as in the genus Nemertodrilus (Eudrilidae), 

 there is an oviduct which can only be compared with that of the 



1 In Sitzungs-Ber. Bohm. Ges. 1889, p. 183. 



