ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 217 



irregularly shaped nucleated corpuscles, from wliicli few or many stiff 

 pseudopodia-liko processes may be extended. The coelomic fluid is of a 

 milky white colour and opaque, and its numerous corpuscles resemble 

 the latter of the two forms found in the blood. 



The nervous system has, in the main, the form usual in earthworms, 

 and its minute structure is very similar. The giant-fibres, which can in 

 no way be called neural canals, are remarkable for the very definite central 

 rod of homogeneous gelatinous material, and for the equally definite 

 inclosing sheath of connective tissue. Prof. Spencer thinks that the 

 most appropriate name for these organs is Vejdovsky's term neurochord. 



The characters of the nephridia are dealt with at some length ; the 

 main features of the system are the presence of numerous nephridia in 

 each segment, the modification of the nephridia in various parts of the 

 body, and the connection of the ducts of the various nephridia. Two 

 distinct kinds are present ; there are numerous small nephridia, which 

 lie so close to one another that the shape of each separate one cannot be 

 distinguished ; each of these consists of a small, somewhat straight tube, 

 and a larger coiled part ; these are present in every segment after the 

 fourth, and are most largely developed in the clitellar region, where 

 they form an almost complete investment for the body-wall, and where 

 each segment has certainly more than one hundred. The second kind 

 of nephridia are much larger, are only present in the posterior region of 

 the body, and occur in the same segments with the smaller kind ; these 

 latter have internal openings, and there is only one pair of them in each 

 segment. The series of gradations which Prof. Spencer was able to 

 make out lead him to the generalization that the specialization of 

 nephridia appears to commence at the posterior end, and to pass gradually 

 forward, the anterior being in a much more primitive condition than the 

 posterior end of the body. The structure of the nephridia is described 

 in detail, and it is especially pointed out that in no part of the body is 

 there any relationship between the nephridiopores and the setee, even 

 when the nephridia become more localized. 



The general characters of the nej^hridia of Chsetopods are discussed 

 at some length, and many interesting questions considered. In dealing 

 with their homologues. Prof. Spencer thinks that it is important to 

 remember that in Chaetopods there is a very clear distinction of the 

 nephridial ducts into two parts — one intracellular, and one intercellular ; 

 the latter leads to the exterior, and has the vesicular part connected with 

 it. It is possible that the former is always mesoblastic in origin, and 

 the latter epiblastic. It is suggested that the various stages in the de- 

 velopment of the nephridia of Chsetopods may be somewhat as follows : — 



(1) A stage (in some Platyhelminth-like ancestor) in which in an 

 unsegmented body a continuous network of nephridial tubules, with 

 flame- or internally ciliated cells, the former uniting to form longitudinal 

 canals leading to the exterior. 



(2) A modification (as seen in DinopMlus gyrociliatus) in which the 

 excretory organs are still in the form of a network with flame cells, but 

 with secondary external openings in each segment, irregularly arranged 

 as in some Planarians, or regularly arranged, as in DinopMlus. 



(3) A further modification, resulting in the formation of numerous 

 irregularly arranged outgrowths from the nephridial network, having 

 the nature of coiled tubules which are directly continuous, and identical 

 in structure with the network. These form the nephridia of the more 



1889. O 



