148 



ZOOLOGY 



of the nephridial network. This extraordiiiary change of a 

 nephridium into a sahvary gland is paralleled in the Arthro- 

 pod Perijoatus, in which developement shows that the salivary 

 glands are modified nephridia. 



Three giant fibres, consisting of a sheath with a clear con- 

 tents, occur dorsal to the ventral nerve cord in nearly all 

 Oligochaets. Connections have been recently traced between 

 them and the nerve fibres. Their function was formerly 

 thought to be solely for the purpose of support ; hence they 

 have been termed the neurochord, and have been compared 

 with the notochord of the Chordata in their physiological 

 action. 



The Oligochaets are hermaphrodite: in the Chaetogasteidae 

 the spermatozoa develope in the coelom, in Lumhricus the 



Fig. 93. — Genital segments of 

 Lumbricws (slightly altered 

 from Howes' Biological A tlas). 

 The left side represents the 

 immature, the right, the 

 mature condition, so far as the 

 male reproductive organs are 

 concerned. After Beddard. 



1. Anterior pair of testes ; the 



second pair are in the 

 next segment. 



2. Seminal vesicles. 



3. Spermathecae. 



4. Vas deferens. 



5. Ovary. 



6. Oviduct. 



7. Eeoeptacula ovorum. 



8. Nephridia. 



9. Nerve cord. 



testes become enclosed in special vesiculae seminales which 

 are outgrowths from three of the septa (Fig. 93). In these 

 vesiculae the spermatozoa mature. 



In the aquatic Oligochaets the ova ripen in the coelom 

 or in an egg sac similar to the vesiculae seminales of Lum- 

 hricus. 



The testes are usually four in number, but there may be 

 only one pair, as in Geoscolex. There is a single pair of ovaries. 



