254 



LECTURE XII. 



108 



connected and constituting one individual. Such individual is due 

 to the succession of incomplete gemmations ; the reproductive process 

 is completed, and the compound animal is resolved into the single 

 individuals, by spontaneous fission. The nucleated cells that are 

 aggregated in the penultimate or proligerous segment, form a signi- 

 ficant feature in its organisation. As long as the anellids propagate 

 by this primitive mode, they manifest no ovaria or testes. These are 

 developed in those individuals w^hich are propagated by gemmation 

 and spontaneous fission, when the proper season for oviparous gene- 

 ration has arrived. M. Edwards observed, that a nereis allied to 

 Myrianes generated in succession six young from the same posterior 

 segment, each of which developed the true generative organs, without 

 the parent itself showing any trace of them. M. Quatrefages has 

 noticed the same fact in a species of Syllis. 



In many of the Anellids, therefore, the phenomena of partheno- 

 genesis are manifested in the immature state ; and, since the indi- 

 viduals so pi'opagated alone acquire the generative organs, an alter- 

 nation of generations may here be afiirmed of such species : the 

 oviparous individuals producing eggs from which the 

 gemmiparous individuals come, and these, in their turn, 

 reproducing the oviparous individuals. 



I next proceed to point out the chief modifications of 

 the generative organs properly so called. And first, as 

 to the generative organs of the leech. In the medicinal 

 species {Hirndo or ISangtdsuga medicinalis) may be seen 

 a number of pairs of little gray rounded bodies, usually 

 nine pairs, which are the testes, Jig. \0S., a a ; these 

 are round whitish sacs, containing a milky fluid, abound- 

 ing in spherical sperm-cells. A little duct proceeds from 

 each to a common longitudinal vas deferens (b), near 

 the anterior fourth of the leecli, which forms a saccu- 

 lated vesicula seminalis (c c), each of which sends its 

 duct to a common small pyriform prostatic gland (e), from 

 which is prolonged a filiform intromittent organ (/"), which 

 projects from the ventral surface of the twenty-fourth 

 ring. But the leech is androgynous, and the female 

 organs are situated behind this prostate and penis, be- 

 tween the seventh and eighth abdominal ganglia ; they 

 consist of two subspherical ovaria {a), two short oviducts 

 (/>), which unite to form a single oviduct {c) that expands into a 

 pyriform muscular uterus {d). 



Brandt and Ratzeburg * found in the bodies which they, with 



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* CLXXXV. 



