320 



C03LHELMINTHES. 



ventral, the latter surrounding the ventral nerve cord. These are 

 connected by a complicated system of capillaries. 



The nervous system consists of brain and ventral cord, the lat- 

 ter containing frequently twenty-three ganglia (the first of five 

 fused, the last of seven). Nerves from the brain go to the eyes. 

 Eight and left of the ventral cord are the hermaphroditic sexual 

 organs. In Hirudo medicinalis (fig. 293) there are nine pairs of 



FIG. 292. 



FIG 



FIG. 292. Hirudo medicinalis^ medicinal leech. (After Leuckart.) a, anterior end 



with three jaws (fc); 6, a single jaw with its muscles. 

 FIG. 293. Nervous system, blood-vessels, sexual organs, and nephridia of a leech, 



ventral view, ft, testes; hb, urinary bladder; ly, lateral blood-vessel; n, ventral 



nerve cord; n/i, epididymis; ov, ovary; p, penis; sc, iiephridia; it, uterus and 



vagina; vd, vas deferens; vg, ventral blood-vessel. 



(7i), the ducts of which unite to form a vas deferens on 

 either side (vd). These pass forward, form by coiling a so-called 

 epididymis (nh) and empty into the median unpaired penis (p). 

 In the space between the epididymis and the first pair of testes 

 are the ovaries (ov) and oviducts and an unpaired vagina (u). The 

 nephridia (17 pairs in this species) are complicated and are pro- 

 vided with bladder-like expansions. 



That the Hirudinei are true annelids and not segmented Plathelminthes 

 is based upon the view that their coelom is reduced by ingrowth of paren- 

 chyma and altered to canals connected with the vascular system. At 

 any rate the ventral and lateral vessels are to be regarded as remnants of 

 a coelom. In Clepsine there are the dorsal and ventral blood- vessels of 

 the Chaetopoda and besides four longitudinal coelomic sinuses connected 

 by transverse anastomoses. The dorsal sinus encloses the dorsal blood- 

 vessel, the ventral many of the viscera, among them the ventral nerve 

 cord. This is also to be regarded as ccelomic, since the nephrostomes con- 

 nect with it. In most Hirudinei a canal system filled with blood has 

 arisen from the coelom and blood-vessel, and in Neplielis is in part lacunar 

 in character. Further relations are shown by Acanthobdella peledina, 

 parasitic on fishes. This has both blood-vessels of the Oligochsetes, a 



