176 



COMPARATIVE ANATOMY. 



surface, which may be derived from the paired organs of similar 

 form in the Annelides. There is either a single pair (Sipunculus), 

 or a few pairs (Thalassema, Sternaspis, Echinrus), in correspon- 

 dence with the slight development of metamerism. Internal open- 

 ings into the body-cavity lie close to the insertion of the tubes 

 into the body- wall, and in several are of service in the generative 

 function, acting as the ducts for the generative products. The 

 greater part of the tube, that is the cascal portion, which lies 

 behind the internal orifice, appears to have an excretory function in 

 the Sipunculidas, and as a rule, is distinguished by its brown colour. In 

 others the whole tube serves merely as a duct for the generative 

 system. In most Gephyrea these organs have each and all similar 

 functions, but in some cases there is a division of labour (Sternaspis), 

 the posterior pair of tubes being in connection with the generative, 

 and the anterior with the excretory, function; so that the difference 

 of function which appears in different genera, is here expressed in 

 the same individual. 



§ 145. 



So far as the structure of the excretory organs is concerned, few 

 fresh characters appear in the Annulata. The organs correspond 

 to the metamerism of the body, for they are regularly distributed on 

 cither side of almost every one of its segments. They have therefore, 

 though Avith but little reason, been called "segmental organs;" a 

 name which is just as suitable for many other organs. Each of 

 them consists of a closely-coiled or loop-like canal (looped canals), 



which has an internal opening, often peculiar 

 in form and always ciliated, and which at 

 the other end opens on to the surface of 

 the body. This canal is sometimes similar 

 in character throughout its whole length, 

 or but slightly differentiated ; frequently 

 several segments may be made out in it, 

 which generally correspond with those 

 already described in the Platyhelminthes 

 and Rotatoria. The innermost portion, 

 on which the opening into the coclom is 

 placed, is ordinarily the longest; it is dis- 

 tinguished by its funnel-like or rosette- 

 shaped orifice (Fig. 83). In the next por- 

 tion the walls may be seen to be glandular 

 in structure. The last portion, which is 

 frequently widened, is provided with a 

 layer of muscle ; it almost always opens 

 on to the ventral surface. These organs 

 are no more purely excretory in function than they are in other 

 Vermes, for we not unfrcquontly find them entrusted with other 

 functions. 



Fig. 83. Internal opening of 



a looped canal of Bran- 



cliiobclolla. 



