III. ANNELIDA: HIRUDIXEI 



2.s:{ 



and a 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 latter con- 

 taining 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 Hlrudo medicina- 



FIG. 271. Hirudo medicinalis, medicinal leech (after Leuckart). a, anterior end 

 with three jaws (k)\ h, a single jaw with its muscles. 



FIG. 272. Nervous system, blood-vessels, sexual organs, and nephridia of a leech, 

 ventral view. /;, testes; hb, urinary bladder; Ig, lateral blood-vessel; n, ventral nerve 

 cord; nh, epididymis; ov, ovary; p, penis; sc, nephridia; u, uterus and vagina; vd, vas 

 deferens; vg, ventral blood-vessel. 



Us (fig. 272) there are nine pairs of testes (//), the ducts of which unite to 

 form a vas deferens on either side (rd). These pass forward, form by 

 coiling a so-called epididymis (nh) and empty into the median unpaired 

 penis (/>). In the jawless leeches the penis is lacking and the sperm, in 

 pointed sprematophores, is inserted in the tissues of the leech. In the 

 space between the epididymis and the first pair of testes are the ovaries 

 (ov) and oviducts and an unpaired vagina (). The nephridia (17 pairs 

 in this species) are complicated canals and are provided 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 parenchyma 

 and altered to canals connected with the vascular system. At any rate the ven- 

 tral and lateral vessels are to be regarded as remnants of a ccelom. In Clepsinc, 

 etc., there are the dorsal and ventral blood-vessels of the Chastopoda and besides 

 four longitudinal coelomic sinuses connected by lacunar spaces. The dorsal 

 sinus encloses the dorsal blood-vessel, the ventral many of the viscera, among 

 them the ventral nerve cord. These sinuses also have flagellated funnels 

 which lead into lymphoid capsules, not, as was formerly thought, into nephridia. 

 In the jawed leeches (apparently by degeneration as is the case in many poly- 

 chaetes) the true blood-vessels are replaced by a canal system derived from the 

 coelomic sinuses, which in Nephilis has in part a lacunar character. For the 



