NATURE 



?6i 



THURSDAY, FEBRUARY. 20, 1902. 



PLANARIANS AND THEIR ALLIES. 

 A Treatise on Zoology. Edited by E. Ray Lankester, 

 M.A., LL.D., F.r"s. Part IV. "The Platyhelmia, 

 Mesozoa and Nemertini." By Prof. W. Blaxland 

 Benham, M.A., University of Otago, New Zealand. 

 Pp. iv + 204. (London : .A., and C. Black.) Cloth, 

 15^. net ; paper covers, i2.f. dd. net. 



IT is a new thing for a whole volume of a zoological 

 treatise to be devoted to the two phyla, Platyhelmia 

 (Turbellaria, liver-flukes and tapeworms) and the 

 Nemertea (long sea-worms), which Prof Benham de- 

 scribes in the work before us. For a number of reasons 

 they are not popular groups of animals. The free-livmg 

 forms are delicate and their very identification is 

 attended with considerable difficulty, and needs 

 mechanical skill and anatomical knowledge. The para- 

 sitic forms add gruesome associations to these troubles, 

 so that it is only the unfortunate patient, the doctor and 

 the scientific agriculturist who have in any sense a 

 working acquaintance with flukes and tapeworms. Very 

 few zoologists know much about them except in such 

 places as Italy, parts of Germany and Egypt, where their 

 unpleasantly common occurrence has created the neces- 

 sity for a thorough investigation of the modes of infection 

 and of the methods for obtaining immunity from their 

 attacks. Hence it is that in most text-books these phyla 

 have not been given that serious and thorough atten- 

 tion which is afforded them In Prof Ray Lankester's 

 " Treatise of Zoology." We have here a compact, lucid 

 and scholarly summary of the anatomy, life-histories and 

 classification of the parasitic flatworms. 



With such an unprecedented amount of room at his 

 disposal Prof Benham would have done well to have 

 brought his opening chapter (on the Turbellaria or 

 Planarians) more up to date in its anatomical, physio- 

 logical and bionomical aspects. It was written and has 

 been in proof some years, as the editor tells us, but we 

 cannot accept the soothing remark that " the editor is 

 satisfied that no important omissions due to this fact 

 occur in the book." \on GrafPs splendid monograph on 

 land-planarians (1899; is barely alluded to, and his 

 striking results on the anatomy and distribution of these 

 animals are omitted, nor are any of his figures intro- 

 duced. The work by Hesse on the eyes of Turbellaria 

 is merely mentioned, but his important results are passed 

 over in silence. The paper by D6rler( 1900) on parasitic 

 VorticidiL', in many ways a striking piece of work, is 

 entirely overlooked. The application, by Rina Monti, 

 of the " Golgi-method " to the detection of nerve-elements 

 of Planarians (1896, 1900), the work by Prof E. L. 

 Mark on Polychaerus (1892 !), the many papers by the 

 school of comparative physiologists and more par- 

 ticularly by Parker and by Prof Loeb, are a few 

 of the more striking omissions which occur to us. In 

 some of these cases we may excuse Prof Benham (but 

 not his editor), since his residence in New Zealand may 

 preclude access to the " Zoologische Jahresberichte " of 

 the last four years ; but even this allowance cannot cover 

 the lack of information on topics such as colour, distri- 

 NO. 1686, VOL. 65] 



bution and variation, upon which the evidence yielded by 

 Turbellaria is not inconsiderable and is of great and 

 growing interest. 



The chapters succeeding that on the Turbellaria are far 

 more satisfactory, though there are several omissions of 

 important or useful papers, such as Looss' work on 

 Egyptian Trematodes and the publications by Dr. Stiles 

 and other members of the Washington Bureau on the 

 parasites of domestic animals. To the student of anato- 

 mical zoology the sections on the " cuticle," " paren- 

 chyma," and the life-histories and their interpretation, will 

 be a clearer guide to the nature of these problems than 

 can be found in any other text-book. 



As to the nature of the cuticle or investing membrane 

 of Trematodes and Cestodes, Prof Benham concludes 

 from the researches of Blochmann and Kowalewski that 

 the outer part of this structure is a true cuticle secreted 

 by epidermal cells. In the adult parasite, however, these 

 cells no longer form an epithelium, and their true nature 

 is further disguised by the fact that they lose their 

 primitive connection with the cuticle and sink into the 

 parenchyma through the basement-membrane pari passu 

 with an outward migration of cells of the parenchyma. 

 These changes are illustrated by an excellent figure from 

 Blochmann's memoir. 



From an economic as well as from a strictly zoological 

 standpoint, the interest of Trematodes and Cestodes 

 centres in the life-history, which becomes progressively 

 complicated in each group. There may be in the history 

 of one parasite two successive larval stages passed in 

 different hosts. Each stage is capable of reproducing 

 many generations of its kind and then suddenly giving 

 rise to the later larval form or (in the case of the latter) 

 to the adolescent stage, which grows to maturity in the 

 final host. The distinguishing feature of the process is 

 the power of multiplication of the parasite while it is still 

 a larva, and it is on the nature of this process that differ- 

 ence of opinion exists. The older writers regarded it as 

 an asexual mode of reproduction, as a kind of budding ; 

 and they emphasised the alternation of these larval asexual 

 generations with the adult sexual generation. The 

 tendency of modern research has been to regard the 

 larval generations, not as asexual, but as produced by the 

 development of unfertilised eggs, in fact as cases of 

 juvenile parthenogenesis ; so that the life-history may on 

 this view be summarised as a series of sexual generations, 

 of which those which occur in the larval stages are 

 parthenogenetic and that which subvenes in the adult 

 worm is either self- or cross-fertilised. 



To this modern view Prof Benham assents, without, 

 however, pointing out the necessity of obtaining critical 

 evidence in its favour. The mature eggs of many animals 

 differ from the body-cells in possessing only half the 

 number of " chromosomes" which characterise the nuclei 

 of the somatic-cells. It ought to be shown whether this 

 holds true of the larval as well as of the adult "eggs" 

 of Trematodes and Cestodes before the view that all the 

 generations are sexual can be considered proved, and it 

 would require still further evidence as to the number of 

 the polar-bodies presumably extruded by the larval 

 eggs before we could accept their parthenogenetic nature 

 as demonstrated. 



The concluding chapters on the Mesozoa and Nemertea 



R 



