i4« 



NATURE 



[June i6, 1898 



a very large variety of sources. The text-books of 

 Korschelt and Heider, Perrier, Lang, Claus, Wasielewski 

 and Bronn's Thierreich have been laid under contribution 

 for cliches, and the author is to be congratulated on the 

 admirable collection he has brought together. The 

 book is intended to be and is as brief as is consistent 

 with an intelligible exposition. Yet it seems hardly 

 possible that Mr. Sedgwick will be able to complete it 

 in another volume of the same size. He has still to treat 

 of the Echinoderma, the entire series of Arthropoda and 

 the Vertebrata (which he would probably call the 

 Chordata). 



There are in the book one or two noticeable and 

 original statements and classificatory innovations which 

 it will be interesting to mention here. Mr. Sedgwick 

 holds, as is well known, special views on the subject 

 of cell-structure. He accordingly defines the Pro- 

 tozoa as "Animals in which there is one nucleus, 

 or, if more than one nucleus, in which the nuclei 

 are disposed apparently irregularly and without re- 

 lation to the functional tissues of the animal. Conju- 

 gating cells of the form of ova and spermatozoa are 

 never formed." In contrast with these the Metazoa are 

 defined as "Animals in which the ordinary (so-called 

 adult) form of the species has more than one nucleus, 

 and in which the nuclei are for the most part arranged 

 regularly and with a definite relation to the functional 

 tissues of the animal (so-called cellular arrangement). 

 Special conjugating individuals of the form of ova and 

 spermatozoa are always formed." 



With reference to this it may be remarked that the 

 nuclei of, say, muscular tissue in Metazoa cannot be 

 shown to have any more definite relation to the functional 

 contractile substance than has the nucleus of a gregarine 

 to its functional contractile substance, and the same 

 kind of remark is true in reference to many other active 

 structures in the two groups compared. 



It surely is not possible to maintain that conjugating 

 cells of the form of ova and spermatozoa are never 

 formed in the Protozoa when we include (as Mr. Sedgwick 

 does) the Volvocinean Flagellata in that group. 



The account of the Protozoa is more complete than is 

 usual in text-books of this size and scope, and the figures 

 of Haemosporidia and Myxosporidia, borrowed from 

 Wasielewski, are particularly good, though the account on 

 •p. 63 of Hcemamoeba Laverani is not quite satisfactory. 

 Mr. Sedgwick, as might be expected from his own 

 important share in elucidating the subject, is very clear 

 and precise in defining the " coelom," and in explaining 

 its real nature. He does not, however, as one could have 

 wished, give the actual history of the word " coelom," and 

 the steps by which the erroneous views of Haeckel, the 

 Hertwigs and other German authorities have been set 

 aside. He says, " formerly the word coelom was used as 

 synonymous with body-cavity or peri-visceral cavity, and 

 no distinction was recognised between the body-cavity of 

 the Arthropoda and the same structure in such forms as 

 Vertebrata." I think it is worth noting that, as a matter 

 of fact, the word ccelom was introduced by Haeckel in the 

 year 1872, in the first volume of his " Kalkschwiimme," 

 p. 468, in the following words : 



"Die wahre Liebeshohle " (contrasted by Haeckel 

 with the digestive coelenteron of Coelentera, to which the 



NO. 1494, VOL. 58] 



term "body-cavity" or " Leibeshdhle " was undesirably 

 applied) " welche bei Vertebraten gewohnlich Pleuro- 

 peritonealhohle genannt wird, und fur welche wir, statt 

 dieses neunsylbigen Wortes die bequemere zweisylbige 

 Bezeichnung Coelom (AcoiXoo/xa, ro, die Hohlung) vorsch- 

 lagen, findet sich nur bei den hoheren Thierstammen bei 

 den Wiirmern, Mollusken, Echinodermen, Arthropoden 

 und Vertebraten." 



For Haeckel the typical coelom was the pleuroperi- 

 toneal cavity of the Vertebrate. At the time when he 

 wrote, that cavity was supposed to have arisen phylo- 

 genetically by a splitting of the mesoblast ; hence the 

 failure of Haeckel to distinguish other cavities, such as 

 the hasmocoel of Arthropoda and of Mollusca from the 

 true coelom. I gather from Hertwig's text-book of 

 Embryology that I was the first to point out that the 

 " schizocoel " (as Huxley called it) of higher Vertebrates 

 could be and should be interpreted (in consequence of 

 Balfour's discoveries in Selachian development) as an 

 enterocffil — a pouch, in this case without lumen— which 

 arises as a solid outgrowth from the enteron, the opening 

 out of its cavity being delayed. Thus the coelom is now 

 characterised by Sedgwick as "a part of the enteric 

 cavity which has lost its connection with that portion 

 which constitutes the alimentary canal in the adult." 

 The enteric pouches of the Actinozoa are " an incipient 

 coelom." Further, it is recognised by Sedgwick that 

 "the coelom, in addition to its mechanical relations, has 

 two most important functions : the one of these is to bud 

 out the reproductive cells, and the other to secrete the 

 nitrogenous waste." The essential cells of the gonads 

 and of the nephridia are parts of the coelom. Mr. 

 Sedgwick's own researches on the development of Peri- 

 patus served more than anything else to establish that 

 the cavity of Arthropods, which I had termed "hoemocosl," 

 is distinct from coelom, and that there is — quite apart from 

 hremocoel— a true coelom in Arthropoda reduced in the 

 adult to nephridial and perigonadial rudiments. My own 

 observations on the pericardium of Mollusca, and on the 

 vascular system of both Molluscs and Arthropods, as 

 well as the work of my pupil Gulland on the coxal glands 

 of Limulus, had tended, before this, to show the exist- 

 ence of " coelom " distinct from " hsemocoel " in both 

 those groups. Thus the erroneous notions promulgated 

 in the " Coelomtheorie " of the Hertwigs were superseded. 

 I am distinctly of the opinion that this step forward — 

 viz. the recognition, definition and characterisation of the 

 true " coelom " as distinct from " haemocoel " — has been 

 due to English observations and English doctrine, and I 

 think that a full account of the history would be valuable 

 to students. 



Mr. Sedgwick necessarily has something to say in this 

 connection concerning the supposed communication of 

 vascular system and coelom in the Leeches. In his 

 excellent account of those animals (in which he not only 

 discusses Acanthobdella, but introduces Kowalewsky's 

 recent figure of its anterior segments) Mr. Sedgwick lays 

 great stress on Oka's recent observations upon Clepsine, 

 and concludes that " we are bound to hold, provisionally 

 at any rate, that in Leeches, as in other animals, the 

 blood system and coelom are separate from one another." 

 I quite agree that there are probabilities in favour of Mr. 

 Sedgwick's conclusion. Twenty years ago, and at in- 

 tervals since then, I have endeavoured to put the matter 



