MEGADRILI 



381 



number of species than any other family of earthworms ; but it 

 is a matter of considerable difficulty to divide the family satisfac- 

 torily into genera. The family as a whole may be defined as 

 having numerous chaetae in most of the segments of the body. 



There is no other definition which will distinguish this 

 family from the next two families, and even this definition is not 

 absolutely distinctive. There are Acanthodrilids which have a 

 large number of chaetae in each segment. The only difference is 

 that in this case in the genus Plagiochaeta the chaetae are 

 implanted in twos ; this is not the case in the Perichaetidae. In 

 all Perichaetidae that are known the sperm-ducts open in common 

 with the ducts of the spermiducal glands ; they generally open 

 into them at some distance from the common external pore. In 

 Megascolex, Perichaeta, and Pleionogaster the nephridia are of 

 the diffuse type so widely spread among these worms, and the 

 spermiducal glands are lobate. Megascolex 

 differs from the others in the fact that in 

 addition to the small scattered nephridia 

 there are a pair of large nephridia in each 

 segment, and the chaetae do not form 

 absolutely continuous circles, but are inter- 

 rupted above and below. Pleionogaster has 

 more than one gizzard but otherwise agrees 

 with Perichaeta; it is confined to the East. 

 Perichaeta is tropical and occurs no doubt 

 introduced in Europe and America. Megas- 

 colex is Old World only, and, like Perichaeta, 

 Australian as well as Oriental. But whereas 

 Perichaeta, is rare in the Australian region, 

 Megascolex is common there. Perionyx and 

 Diporochaeta are the other genera which it 

 is possible to recognise. Both of them have 

 paired nephridia, and neither of them have 

 intestinal caeca, a peculiarity which they both 

 share with Megascolex and Pleionogaster. 

 Perionyx principally differs from Diporochaeta 

 in that the spermiducal glands are lobate, whereas in the latter 

 they are as in the Acanthodrilidae. Perionyx is Oriental; 

 Diporochaeta occurs in Australia and New Zealand. 



A very distinctive feature of Perichaeta perhaps only of the 



- 196. Perichaeta 



ereretti F. E. B. x 1. 



spt spermathecai 



makpore. 



