156 HELMINTHOZOA. 



but when it arrives at the sheath of the proboscis it becomes enclosed 

 in a special canal, and thus penetrates into the cephalic region, where 

 it bifurcates to form the two lateral trunks, & Jc, which, after forming 

 almost a complete circle around the cerebral ganglia, anastomose with 

 the lateral vessels, i i. 



(415.) The vessels of the body thus becoming conjoined at the points 

 h h, form a loop, g g, which surrounds the cephalic region, and is of 

 considerably greater calibre than the trunks from which it is derived. 



(416.) The fluid contained in the vascular system above described is 

 generally colourless, but sometimes of a reddish or yellowish tinge ; it 

 is, however, completely devoid of blood-corpuscles, resembling, in this 

 respect, the blood of the Annelidans, which will be desciibed on a future 

 occasion. 



(417.) In these worms, the central or abdominal space is separated 

 from the two lateral cavities by vertical septa, to which the reproduc- 

 tive organs are attached. The latter consist of a series of caeca (fig. 76, 

 n n), which are so exactly alike in the two sexes, that it is impossible to 

 distinguish the male from the female, except by their contents. These 

 caeca are apparently formed of several layers of cells, and are covered 

 externally with vibratile cilia. Except during the period of reproduc- 

 tion, they contain, in both sexes, nothing but an opaline fluid ; but when 

 called into action, the ovaria in the female are found to be filled with a 

 liquid wherein corpuscles of various shapes are seen to be suspended. 

 Some of these have the appearance of perfectly diaphanous spheres, 

 which are sometimes isolated, at others surrounded with granulations ; 

 and likewise oil-drops of a beautiful golden colour may be detected : but 

 the ovaria never seem to contain perfectly developed ova ; these are only 

 met with in the lateral cavities of the body, where they may frequently be 

 seen, in different stages of development, floating about between the caeca. 

 The vitellus and Purkinjean vesicle are always more or less apparent. 

 When these eggs have acquired their full growth, they stuff, so to speak, 

 the whole body of the worm, pressing upon the alimentary canal to such 

 a degree that it seems in danger of becoming atrophied, and almost 

 completely effacing the median abdominal cavity ; nay, so closely are 

 they squeezed against each other, that they lose their spherical form 

 and become polygonal. The number of these eggs is prodigious ; and 

 M. de Quatrefages estimates seven or eight thousand to be a moderate 

 complement for a middle-sized worm of this description. 



(418.) The male Nemerteans present phenomena very similar to those 

 just described as occurring in the female. At the period of reproduc- 

 tion, the testicular caeca become filled with granulations of various sizes, 

 sometimes isolated, sometimes grouped together in round masses ; but 

 these do not contain spermatozoids. The latter seem only to occur in 

 the lateral cavities of the body ; and even there they are found in various 

 stages of development, from granular masses, such as are met with in the 



