OLIGOCHAETA 289 



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 of 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. Th^ 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 

 Eutyphoeiis, where the end 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 

 obvious that the mechanism of copulation is far more complicated in 

 the Lumbricidae. Here the worms come into contact along their 

 ventral surfaces and each becomes enveloped in a mucous sheath. Close 

 adhesion is secured between the clitellum of one worm and the seg- 

 ments 9 and 10 of the other, partly by embracing movements of the 

 clitellum and partly by the chaetae of the same region being thrust 

 far into the body wall of the partner. The sperm passes out of the 

 male aperture and along the seminal groove to the clitellum ; how it 

 enters the spermathecae of the other worm has never been observed. 



The cocoons are formed some time after copulation. The worm 

 forms a mucous tube as in copulation. The cocoon is then secreted 

 round the clitellum and finally the albuminous fluid which nourishes 

 the embryo is formed between the cocoon and the body wall and the 

 worm frees itself from the cocoon by a series of jerks. All three 

 products, mucus, cocoon substance and albumen, are secreted by the 

 clitellum and each probably by a distinct type of cell. The eggs are 

 sometimes extruded and passed backwards into the cocoon while it 

 is still in position on the clitellum but the spermathecae eject the 

 spermatozoa when the cocoon passes over them. 



The embryo oi Lumbriciis is illustrated in Fig. 200. The prototroch 

 is absent but the gut and stomodaeum are developed early to absorb 

 the albumen in the cocoon. There are two mesoblast pole cells at the 

 hinder end which bud off the mesodermal strips: there are three 

 ectodermal pole cells on each side, the ventralmost a neuroblast 

 forming half the nerve cord and the two others nephroblasts giving rise 

 to longitudinal rows of cells which divide up to form the nephridia. 



A primitive kind of nephridium in the Oligochaeta is that de- 

 scribed in Lumbricus, of which there is a pair for each segment, the 

 nephrostome projecting through the septum and opening into the 

 cavity of the segment in front. A great many modifications of this 



