316 ANNELIDA 



are wanting. Acantliohdclla, 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 Oligocliceta. 

 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 subtermilial 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 Rhynchohdellidae, 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 



Fiff 497 A 



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

 leech ; B, ante- of paired nephridia in the middle portion of the body 

 rhynchobdeiiid (seventeen pairs in Rirudo), the inner ends opening into 

 l.^^chitiu^oVs the sinuses representing the body cavity. The nervous 

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

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

 anterior ganglion pairs are fused together forming an 

 infra-oesophageal 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 foim 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 foimed by a elitellum on the ninth, tenth, and 

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

 parent. 



