316 ANNELIDA 



are wanting. Acanthobdella, a Russian fresh-water leech, is an exception 

 to this rule, having setae on the first five somites. Paired gills are present 

 in a few genera. The head is not distinctly marked off from the trunk 

 but the prostomium projects in front of the mouth, as in the Oligochata. 

 The body cavity differs in character from that in most other annelids in 

 that it is filled secondarily with a vacuolated parenchyma; several tube- 

 like spaces are, however, still left in it which are connected with the 

 vascular system and contain blood. 



The mouth is ventral or subtermihal in position and opens into a 

 pharynx which is provided with salivary glands and in turn leads into 

 the oesophagus and the targe crop; this organ has paired segmental 

 pouches and passes back to the stomach, which may also be provided 

 with paired pouches. The short intestine passes to the 

 anus at the hinder end of the body above the sucker. 

 The pharynx (Fig. 497) is provided with three serrated 

 chitinous plates in the medicinal leech and its allies, by 

 means of which the animal may draw blood from the 

 body of its host. The Rhynchobdellidae, on the other 

 hand, have no such plates but a proboscis which can be 

 thrust out of the mouth and be made to pierce the skin 

 of another animal. The main vascular system consists 

 in general of four longitudinal blood vessels, a dorsal, a 

 anterior end~o ventral, and two lateral. The excretory system consists 

 feech 1 ; 6 B, C ante- of paired nephridia in the middle portion of the body 

 ?hynchobdem<f (seventeen pairs in Hirudo), the inner 'ends opening into 

 i, US cMtin e ous the sinuses representing the body cavity. The nervous 

 tended probos- system is like that of other annelids. The two longi- 

 suck'er. 8 ' ral tudinal nerves are close together and several of the 

 anterior ganglion pairs are fused together forming an 

 infra-03sophageal ganglion. The special sense organs consist of a number 

 of pairs of eyes (in Hirudo five) at the forward end of the body and sense 

 buds which are most numerous at the forward end. 



The leeches are hermaphroditic. A number of pairs of testes alter- 

 nate usually with the segmental lateral pouches of the digestive tract 

 and communicate with a pair of vasa deferentia which proceed to the 

 male genital pore in the anterior part of the body, joining to form a 

 penis at their anterior end. The female pore lies just behind the male; 

 two ovaries are present which are joined by the oviducts with the vagina. 

 Fertilization is effected by means of a spermatophore and the eggs are 

 usually laid in a cocoon formed by a clitellum on the ninth, tenth, and 

 eleventh somites. The young animal is born with the form of the 

 parent. 



