144 



LECTURE XII. 



It sometimes happened that both ends of a segment reproduced a 

 tail. Wishing to ascertain if the vegetative power was inexhaustible, 

 Bonnet cut off the head of one of these worms, and, as soon as the 

 new head w as completed, he repeated the act ; after the eighth 

 decapitation the unhappy subject was released by death, — the ex- 

 ecution took effect, — the reproductive virtue had been worn out : 

 this series of experiments occupied two summer months. Since many 

 of the smaller kinds of worms and Naids frequently or habitually 

 expose a part of their body, the rest being buried in the earth, both 

 they and their enemies profit by the power of restoration of the 

 parts which may be bitten oflT. 



With this power of reproduction of lost extremities is associated 

 that of spontaneous fission in the genus Nai.s. In these little red- 

 blooded worms, the last joint of the body gradually extends and 

 increases to the size of the rest of the animal : its anterior part 

 begins to thicken and to be marked off by a deeper constriction from 

 the penultimate joint. In the JVais jjroboscidea a proboscis shoots 

 out from it, like that on the head, and it is then detached from the 

 old Nais. It often shoots out, previously to its separation, another 

 young one from its own last joint in a similar way, and three 

 generations of Naids may thus be seen organically connected, and 

 forming one compound individual. Distinct sexual organs are 

 developed in the Nais for both the formation and fertilisation of ova : 

 but the illustrations of the generative system in the preparations 

 ri^ before you are derived from the leech, the earth-worm, 

 and a few of the dorsibranchiate Anellides. 



The medicinal leech is androgynous, like the rest of 

 the Anellata; it has nine pairs of testes {Jig. 15, a, a), 

 two vasa deferentia (6, h), and two sacculated vesiculse 

 seminales (c, c), which send their ducts to a common 

 prostatic body (d), from which the penis (e) is continued : 

 this filament projects from the middle of the ventral sur- 

 faces of the twenty-fourth ring, between the sixth and 

 seventh ganglion : a distinct slender ejaculatory tube is 

 continued along the middle of the intromittent organ. 

 Each testis is a round whitish sac, 

 containing a milky fluid abounding 

 with clear spherical corpuscles : a 

 short duct carries the secretion to the 

 common longitudinal vas deferens. 

 The female organs (Jig. 76.) lie be- 

 tween the seventh and eighth ner- 

 vous ganglia, and consist of two sphe- 

 rical ovaria (a) and two short oviducts 



