172 COMPARATIVE ANATOMY. 



charactei' also distant relations to what obtains in tlie Annelides are 

 expressed. The most important difference consists in tlie limitation 

 of the transverse anastomoses, -which surround the intestines, to one 

 only, which is consequently very much modified. Here, then, again 

 the characters exhibited are in correspondence with a rudimentary 

 metamerism. More or less extended portions of the vessels serve as 

 organs for the movement of the blood, and are very different in 

 various genera. 



§ 141. 



The nutritive fluid forms the contents of the coelom, as well a,s 

 of the vascular system; its form-elements are generally slightly 

 differentiated cells. When the vascular system is marked off from 

 the coelom its contents are known as blood. The form-elements in 

 many Annelids are colourless. In many Nemertina the blood-cells 

 have a red colour (Borlasia) ; the fluid is coloured in many Annelids, 

 and this colour is occasionally green, but more frequently red. In 

 several cases the form-elements contain the colouring matter. The 

 plasma also may be definitely coloured, as, for example, in the 

 Lumbricidse. When the vascular system is separate, the contents 

 of the body-cavity are generally indifferent ; we then find a peri- 

 visceral fluid (known also as chyle) in addition to the blood ; this is 

 always uncoloured. When the vascular system is atrophied, the 

 fluid filling the coslom is frequently of a red colour (Glyceridee), like 

 the blood of other forms. 



Excretory Organs. 

 § 142. 



It is quite uncertain what is the functional significance of a very 

 large number of organs comprised under this head ; of others, how- 

 ever, it is certain that their secretion is essentially similar to that of 

 the renal organs of higher animals. But they all have a number 

 of common relations to the organism, which are of weight, although 

 the organs are so differently connected that it is impossible to 

 completely prove their homology. 



In its more developed forms the excretory apparatus is a system 

 of simple or branched canals, which opens to the exterior on the 

 surface of the body, and is provided with internal orifices when a 

 coelom is distinctly differentiated; when it is not, the ends of the 

 tubes, or the finest branches of the canals, are closed. When the 

 body is not segmented there is a single pair of these organs ; when 

 metameres are formed this system is segmented. The indifferent 

 condition of the excretory organs is represented by a pair of 

 caecal tubes, differentiated from the integument, and therefore 

 derived from the ectoderm. Organs of this kind, which open behind 

 the head, are known in the Nemertina, but the canal system said to 



