520 



ANNELIDA. 



Ehynchobdellidae ; on the contrary, Oka has shown that in Clepsine no such 

 continuity exists ; but for the Gnathobdellidae we have the statements of Bourne 

 and Lankester that they have seen botryoidal tubes and small blood vessels in 

 direct continuity, and the statement of Burger, in direct opposition to the above, 

 that in the young Nephelis the two systems are separate, and contain differently 

 coloured fluids. 



In weighing these statements, we must not forget (1) that it is extremely 

 unlikely that animals so closely similar in other respects as the Gnathobdellidae 

 and Hhynchobdellidae should differ in such an important point of structure ; (2) 

 that the sinus network and the small blood vessels are intermingled in the 

 greatest complexity of arrangement, and that the walls of both have a very 

 similar structure, and that they contain a very similar fluid, and therefore might 

 very easily be mistaken for one another, and (3) that a continuity between the 

 vascular system and the undoubted coelom of the sinuses would be a unique 

 phenomenon in the structure of the animal kingdom. The bearing of these 

 considerations is obvious, their weight indubitable ; and it is clear that until 

 more detailed and elaborate observations in support of the view of continuity 

 are forthcoming, we are bound to hold, provisionally at any rate, that in the 

 Leeches, as in other animals, the blood system and coelom are separate from one 

 another. This position is still further strengthened when we remember that 

 developmentally the two systems are separate, and if continuity exists it is only 

 established comparatively late in life, after the larval and embryonic period. 



vl 



/3 nphg dr 



FIG. 421. Diagrammatic representation of a nephriclium of Clepsine (after Oka), a the 

 anterior division ; /3 the posterior division ; 7 the median division ; bl the invagination of 

 skin ; caps capsule ; dr glandular cells of the nephridium ; lab labyrinthic coil of nephridial 

 tube ; n nerve cord ; nphg duct of nephridium ; si lateral sinus ; tr funnel ; vl ventral sinus. 



The nephridia vary in number in 'the different genera, and even in 

 species of the same genus. They are absent from the anterior and 

 posterior part (Fig. 424), and are best developed in the middle 

 region of the body. They consist (Fig. 421) of a long convoluted 

 canal, which opens at one end to the exterior on the ventral surface 

 of the body, and at the other internally into some part of the sinus 

 system (see above, p. 518). The part of the organ next the external 

 opening (sometimes as in Hirudo dilated into a vesicle) is probably 

 derived from an invagination of the skin, and is intercellular; the 



