1898] SOME NEW BOOKS 55 



mere,' because some authorities have considered these parts to repre- 

 sent distinct individuals in an animal cult my. 



This volume contains a vast amount of information, yet it is 

 inevitable that the various groups should lie treated somewhat un- 

 equally. The parts on the Porifera and Coelentera, for instance, are 

 lather dry. Opportunities are lost for interesting comparisons. With 

 regard to the sponges, no discussion of affinities is given, the import- 

 ance of the collar-cells and of the development is not sufficiently 

 brought out; within the group itself, the significance of the Olynthus 

 si age of development as representing a possible ancestral condition is 

 not appreciated. Similarly, the relationships of the various Coelen- 

 terate groups aie not discussed, and the resemblances between the 

 Aralephae and Actinozoa (Anthozoa) seem to us scarcely enough 

 insisted upon. Xo adequate discussion of the affinities of the Cteno- 

 phora can be found, they are placed in the Phylum Coelentera, and a 



reader of this 1 k might never suspect that many authorities separate 



them entirely from the ( 'oelenterates. Codoplana and Ctenoplana are 

 merely mentioned. 



The Platyhelminths, Nemertines, Nematelminths, and Rotifers are 

 placed outside the true Coelomata. A full discussion of the homology 

 of the coelom is promised in the general part of the next volume ; for 

 the present we are given the dogmatic statement that " the coelom is 

 derived from the enteron," a statement which cannot fail to confuse, 

 if not mislead, the student, since in the two phyla (Mollusca and 

 Annelida) next described this is not the case. 



In matters of theory there will always be divergence of opinion, 

 and we cannot help thinking that the student would have gained a 

 nution of the nature of the coelom far clearer and nearer the truth, if 

 Mr Sedgwick, in this volume at all events, had merely, without theo- 

 retical bias, stated the plain matter of fact : that from the Platyhel- 

 minths upwards we find a cavity in the mesoblast into which the 

 genital cells are shed, and from which they are led to the exterior by 

 special ducts, that in the lower forms this cavity is small, whilst in 

 the higher it becomes large, and forms what we call the coelom. 



In the succeeding chapters, the relations of the coelom to the 

 haemocoele, and its division in the Mollusca into pericardial, renal, 

 and genital regions, are all explained with admirable clearness. 



As usual in text-books, the terminology of the excretory organs 

 is somewhat confused. The term nephridium is not applied to the 

 excretory ducts of the Platyhelminths, but to the renal coelom of the 

 Mollusca, and the segmental excretory organs of the earthworm. We 

 are further told that " the renal organs, called nephridia, are part of 

 tin' coelom," i.e., according to the author's theory, of the enteron. Now, 

 is there a particle of evidence to show that the nephridia of the earth- 

 worm have ever formed a part of the enteron, or even of the coelom ? 



In the excellent chapter on the Annelida two things may be 

 noticed. Discussing the homology of the prostomium, Mr Sedgwick 

 argues that it probably corresponds to a real segment, since mesoblastic 

 somites are found in front of the mouth in Peripatus. Surely we 

 should not argue from the Arthropod to the Annelid, but from the 

 Annelid to the Arthropod. The evidence, when taken in its proper 

 order, leads us to an entirely opposite conclusion. It is strange, also, 



