198 THE ANATOMY OF INVERTEBRATED ANIMALS. 



by commissural cords with the anterior ganglia of the chain, 

 which extends through the whole length of the body on the 

 ventral wall of the perivisceral cavity. 



There are no eyes, nor are any other organs of special 

 sense known. 



The Earthworm is hermaphrodite. The testes are two 

 pairs of large sacs, each of the anterior pair being bilobed. 

 The testes of opposite sides are united in a common median 

 reservoir, situated in the tenth and eleventh segments, from 

 which, on each side, ducts take their origin. The two ducts 

 of the testes of the same side unite into a single vas deferens, 

 and these two vasa deferentia open externally on the ventral 

 aspect of the fifteenth segment. The ovaries are two minute 

 solid bodies, not more than -^ of an inch long, attached to 

 the posterior face of the mesenteric septum which separates 

 the twelfth and thirteenth segments. They therefore lie in 

 the cavity of the latter. The oviducts are quite distinct from 

 the ovaries, and open internally by wide, funnel-shaped aper- 

 tures, situated in the cavity of the thirteenth segment. From 

 these funnel-shaped ends the oviducts are continued, as 

 slender tubes, through the mesenteric septum which separates 

 the thirteenth from the fourteenth segment, and open on the 

 ventral face of the latter. 



Four globular spermathecse, or receptacles of the sper- 

 matozoa, are situated, two on each side, in the tenth and 

 eleventh segments, and open on the ventral face between 

 the ninth and tenth and the tenth and eleventh segments 

 respectively. These are filled when copulation takes place, 

 during which process the two worms are said to be bound 

 together by a tough secretion of their clitella. 



The development of the Oligochceta has recently been 

 carefully investigated by Kowalewsky. The eggs of the 

 Earthworm are laid in chitinous cocoons or cases, which are 

 probably secreted by the clitella. In addition to the eggs, 

 the cocoons inclose an albuminous fluid, and packets of sper- 

 matozoa. The vitellus is invested by a membrane, and con- 

 tains a germinal vesicle and spot. Complete yelk-division 

 takes place, and eventually the blastocoele becomes reduced 

 to a mere cleft. The blastomeres are disposed in two layers 

 one consisting of small and the other of large blastomeres. 

 The embryo thus formed becomes concave on the side formed 

 by the large blastomeres, until it assumes the form of a sac, 

 ciliated externally, with an opening, the future mouth, at one 

 end ; the cavity of the sac being the primitive alimentary 



