320 



CCELUELMINTIIEB. 



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

 connected by a complicated system of capillaries. 



The nervous system consists of l)rain and vejitral 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. 

 Right and left of the ventral cord are the hermaphroditic sexual 

 organs. In Hirudu medicinalis (fig. 203) there are nine pairs of 





Fig. 293. 



Fig. 293. 



Fig. 292. — Hirndo medicinalis, medicinal leech. (After Leuckart.; a. anterior end 



with three .laws (fc); 5, a single jaw with its muscles. 

 Fig. 298. — Nervous system, blood-vessels, sexual organs, and nephridia of a leech, 



ventral view, h, testes; hb, urinary bladder; Ig, lateral blood-vessel; 71. ventral 



nerve cord; nh, epididymis; oi', ovary; p, penis; sc, nephridia; n, uterus and 



vagina; u(/, vas deferens: I'y, ventral blood-vessel. 



testes (//), the ducts of which unite to form a vas deferens on 

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

 epididymis (iiJi) and empty into the median unpaired penis (/)). 

 In the space between the epididymis and the first p)air of testes 

 are the ovaries (oi>) and oviducts and an unpaired vagina (?(■). 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 ccelom is reduced by ingrowth of paren- 

 oliyma 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 crulom. In Clepsi7ie there are the dorsal and ventral blood-vessels of 

 tlie Chsetopoda and Vjesides four longitudinal crelomio sinuses connected 

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

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

 coi'd. This is also to be regarded as cailomio, since the nephrostomes con- 

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

 arisen from thecoelom and blood-vessel, and in JS'epheHs is in part lacunar 

 in character. Further relations arc shown by AcanthobdelJa j)e!ediiia, 

 parasitic on lishes. This has both blood-vessels of the Oligochtetes, a 



