3/8 OLIGOCHAETA 



chaeta have been placed by some systematists in a separate family. 

 The first named contains four species which are fairly common. 

 It is one of those worms in which the chaetae are not exactly 

 related to the segmentation of other organs, which moreover 

 sometimes show an independence in their segmentation ; thus 

 there are more nerve ganglia in the anterior segments of the body 

 than there are septa. 



FAM. 6. Tubificidae. The worms belonging to this family 

 are of small size, and are all inhabitants of fresh or salt water, or 

 the margins of pools and the sea. They differ from the last family 

 in that asexual reproduction never occurs, and that the reproduc- 

 tive organs are situated rather farther back in the body. The 

 male pores are upon segment 11, and the oviduct-pores upon the 

 following segment. This family differs from the Lumbriculidae 

 in the fact that there are only a single pair of sperm-ducts. 



The earliest known Tubificid was the common Tubifex rivu- 

 lorum, so widely dispersed in this country and elsewhere ; but 

 with it was at first confounded the somewhat similar genus 

 Limnodrilus, which only differs in that the chaetae are all of the 

 cleft variety, and never capilliform, as in Tubifex. The genera 

 are mainly distinguished by the characters of the chaetae and of 

 the male ducts. At the base of the series perhaps lies Ilyodrilus, 

 which has many points in common with the Naids. The form 

 of the terminal chamber into which the sperm-duct opens has the 

 same simplicity as in that group, and the intestine is surrounded 

 with a network of blood-vessels as in the Naids, a structure which 

 is otherwise wanting in the Tubificidae. The development of 

 the ova also is upon a plan which is met with in the Naids. 

 The atrium (see p. 361) becomes more complicated in other 

 Tubificidae. The extremity also is as a rule modified into a 

 retractile penis. The discrete " prostate," of which we have 

 already spoken, marks out a considerable number of genera, such 

 as Tubifex, Limnodrilus, Spirosperma, Hemitubifex. In the marine 

 Clitellio there is no such structure at all, and it is also wanting 

 in the South American Hesperodrilus. In JBranchiura there is a 

 complete prostatic investment of the atrium, and in Telmatodrilus 

 a large number of separate aggregations forming as many distinct 

 prostates. Vermiculus, a genus consisting of but one species, 

 found by Mr. Goodrich on the sea-shore in the neighbourhood 

 of Plymouth, is remarkable for the unpaired character of the 



