254 THE INVERTEBRATA 



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. i88 C) consists essentially of two 

 pairs of testes in segments lo and ii and one pair of ovaries in 

 segment 13, followed by ducts which open by large funnels just behind 

 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 oi spermathecae 

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

 external aperture there are sperniidiical {prostate) glands which do not 

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

 (nephromixia) 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 

 to be arranged in a complete ring round the body {perichaetine). The 

 dorsal pores may disappear. The oesophagus may lose its calciferous 

 glands and the gizzard may be absent or develop into several. The 

 reproductive organs vary in small but important particulars. There 

 are nearly always two pairs of testes in segments 10 and 11 and one 

 pair oi ovaries in segment 13, but the testes may be reduced to a single 

 pair. There are usually two pairs of spermathecae but the number 

 varies and occasionally they are absent altogether. Tht prostate glands 

 (of unknown function) are nearly always present in earthworms 

 except in the Lumbricidae. 



The simplest method of copulation in earthworms is that found in 

 Eutyphoeus, where the ends of the sperm duct can be everted to form 

 a penis. This is inserted into the spermathecal apertures and the 

 spermatozoa thus pass directly from one worm to another. It is 



