OLIGOCHAETA 287 



types, the ''earthworm", a larger burrowing terrestrial form, and the 

 aquatic oligochaet which is much smaller and simpler in structure. 

 It is probable that the former type is the more primitive ; the aquatic 

 oligochaet shows many characters which resemble those of the archi- 

 annelids and are most likely due to a process of simplification. The 

 reasons for the conclusion that the aquatic oligochaets are not the 

 oldest of these groups are given below. 



The Earthworms 



These are divided into a number of families of which the most 

 important are the Lumbricidae, containing Lumbricus and Allolobo- 

 phora^ and the Megascolecidae which is the largest of all. 



The primitive forms in all families resemble Lumbricus in the 

 following characters. There are a large number of segments and each 

 one is furnished with eight chaetae arranged in pairs and all on the 

 ventral side of the worm. A series of dorsal pores is found along the 

 back in the intersegmental grooves. The alimentary canal is cha- 

 racterized by a large muscular pharynx by which the food is sucked 

 in, with many glands, the secretion of which is used in external 

 digestion. The oesophagus in one part of its length gives rise to one 

 or more pairs of diverticula, the cells of which secrete carbonate of 

 lime {oesophageal pouches and glands). At the end of the oesophagus 

 or the beginning of the intestine there is a thick-walled gizzard in 

 which the food is masticated with the aid of the soil particles. The 

 intestine has a dorsal ridge, the typhlosole^ to increase the absorptive 

 surface. The nervous, muscular and circulatory systems exist through- 

 out the earthworms with little variation from the condition in 

 Lumbricus. 



The reproductive system (Fig. 199 C) consists essentially of two 

 pairs of testes in segments 10 and 11 and one pair of ovaries in 

 segment 13, followed by ducts which open by large funnels just be- 

 hind the gonads and discharge to the exterior in the next segment in 

 the case of the oviduct, and several segments behind in the case of the 

 sperm duct. The testes, at least, are enveloped by sperm sacs (vesiculae 

 seminales) which are outgrowths of the septa, and in the cavity of 

 these the sperm undergo development. In some earthworms there are 

 no sperm sacs and this condition, resembling that in the Polychaeta, is 

 probably the earliest in the group. There are two pairs of spermathecae 

 in the region in front of the testes. In the neighbourhood of the male 

 external aperture there are spermiducal [prostate) glands which do not 

 actually open into the sperm duct. A single pair of segmental organs 

 (open nephridia) is present in each segment. 



The variations which occur in more specialized members of all 

 families are as follows. The chaetae may increase in number and come 



