236 COMPARATIVE ANATOMY chap. 



gives off, along its whole length, branches which again ramify. The 

 longitudinal canals and their branches lie neither on nor in the blood 

 sinuses, but are directly imbedded in the gelatinous connective tissue 

 (body parenchyma). The longitudinal canals either open externally 

 through several lateral efferent ducts, or on each side by one lateral 

 canal, which may branch off from the longitudinal trunk either 

 anteriorly, in the middle, or posteriorly. 



The nephridial system of the Schizonemcrtina and Hoploiwrmrtiiut shows a certain 

 agreement with that of the Flatocles. Here, as there, we meet with lateral longi- 

 tudinal trunks which open externally on each side, either through one aperture 

 (e.g. in the Platodes among the Rhahdoaelida : Derostoma, Prorhynclnis, Gyrator, 

 Mesostoma ; and among the Trematoda : Polystomidce), or through numerous lateral 

 ducts in more or less segmental arrangement (e.g. in Platodes among the Triclada). 

 The longitudinal trunks of the Neinertina may even, as in the Platodes, be broken 

 up into a plexus, or they may be present in numbers. Wliere the nephridial system 

 in the Neinertina (Hoplonemertina)' is neither in direct nor indirect communication 

 with the blood- vascular system, it belongs to the branched type, as in the Platodes, 

 which have no blood-vascular system. In the Ncmcrtina, it is true, the nephridial 

 system lies only in the front or foremost portion of the body. Terminal excretory 

 cells have not been proved to exist, and the canals are lined with a ciliated epi- 

 thelium, while in the Platodes (everywhere and in all divisions ?) each canal runs 

 ivithin cells arranged in a single row (intracellular). These differences, however, 

 ought not to prevent a recognition of the homologfy between the excretory systems 

 of the Platodes and the Neuurtina. 



Nemathelmia. — The Nematoda and Acanthocephala must be 

 described separately. In the first, longitudinal canals occur in the 

 lateral lines ; till now no inner lining of epithelium h^s been proved 

 to exist. The two longitudinal canals unite at the anterior end 

 of the body to form a longer or shorter unpaired canal, which opens 

 externally near the brain by a ventral median pore. The homologies 

 of these canals, which are considered to be excretory canals, are quite 

 uncertain. In the Gordiidce they are wanting. The canal-like space 

 (section of the body cavity) which surrounds the intestine of these 

 animals is said to divide (in Gardius Preslii) in front of the cloaca into 

 two branches, and these perhaps open into the cloaca. Whether this 

 canal is an excretory tube is, however, quite uncertain. 



In the Acanthocephala, in the subcuticle of the integument, a 

 system of canals is found which will be described in the section on 

 the vascular system. The anterior part of this canal system, which is 

 quite separated from the posterior part, was formerly claimed as an 

 excretory system ; but there are difficulties in the way of accepting 

 this view, chiefly because it has no external aperture. 



Annulata. — The following scheme may be given as of general 

 application to the excretory or nephridial system. It consists of 

 paired tubes (nephridia), open at both ends, which are repeated 

 segmentally. Each nephridium is in open communication with the 

 body cavity or blood sinuses by an inner aperture ; the external 

 aperture lies in the integument. The nephridia therefore form an 



