Dec. 18, 1884] 



NA TURE 



[ 47 



new. In one genus alone, that of the Reteporce, the 

 number of known species has been raised by the dredgings 

 of the Challenger from 31 to between 50 and 60. 



The determination and definition of species in a collec- 

 tion so large as that of the Challenger Polyzoa, and in a 

 group of organisms in which the differences are far from 

 being always strikingly obvious, cannot but be a work of 

 labour. The critical examination of the species in 

 such a genus as Retepora, for instance, which is repre- 

 sented in the Challenger collection by 23 species, and 

 Cellepora, which is represented by no fewer than 31, 

 requires no ordinary patience, and the author must be 

 congratulated on having so far brought to a conclusion 

 labours which, in order to be conscientiously performed, 

 must be often wearisome and monotonous. 



Among the most important contributions of the Report 

 to the systematic zoology of the Polyzoa is the revision 

 which it contains of Adeona and allied genera. A critical 

 comparison of the species of Adeona with species belong- 

 ing to other genera which had been hitherto placed among 

 the Escharidce has necessitated the founding of a new 

 family, Adeoneec, in order to include the whole in a single 

 natural group. This family has several peculiarities, 

 among which the most interesting is the possession by all 

 the species of three different kinds of cells, which the 

 author terms zocecial, ocecial, and avicularian. Ocecia of 

 the ordinary type are entirely absent, and their function 

 appears to be performed by special cells which differ in 

 form from the others. When decalcified these ocecial 

 cells appear as thick-walled sacs, containing in most 

 cases an ovoid mass, which resembles the contents of an 

 ordinary ocecium, and like these is almost certainly 

 embryonal. Mr. Busk has further made the important 

 observation that in some of them there is lodged instead 

 of this mass a polypide similar to those inhabiting the 

 zocecial cells, and he concludes that the embryonal 

 mass is derived from a polypide, which it finally 

 replaces. 



Among other peculiarities of the Acleoneae is one 

 which, notwithstanding its apparent triviality, derives 

 importance from its constancy. This consists in the 

 universal presence of a projecting point at each end of 

 the base in the avicularian mandibles both large and 

 small. In doubtful fragments this character alone will 

 often indicate the affinities of the species. 



The descriptions of the new species are throughout the 

 work drawn up with that care and precision which charac- 

 terise all Mr. Busk's zoological writings, while the absence 

 of redundant description and the exclusion of characters 

 not necessary for the diagnosis, give to his definitions a 

 conciseness which will be appreciated as it deserves by 

 all who require to consult the Report. 



In a large proportion of the diagnoses the author has 

 had recourse to the chitinous elements of the skeleton. 

 These are the so-termed opercula or oral valves, and the 

 chitinous parts of the avicularia and vibracula ; and a 

 very large number of accurately-executed outlines are 

 given in order to show the various forms assumed by 

 these elements in the different species. The employment 

 of the chitinous elements in the classification and descrip- 

 tive zoology of the Polyzoa is due entirely to Mr. Busk, 

 who has convinced himself that " their value for these 

 purposes cannot be over-rated, while their importance 



extends far beyond the mere distinction of genera and 

 species." 



The descriptions of the species are of course necessarily 

 confined to the hard parts, whether calcareous or chitinous, 

 for, except in living examples, it is rarely possible to 

 determine any facts of importance regarding the soft 

 parts of the colony. The author, however, gives two 

 highly instructive figures of the avicularia of Bicellaria 

 mini, in which the muscular apparatus and other 

 soft parts of these curious and still enigmatical bodies are 

 clearly and beautifully represented. In one of his figures 

 of Carl ! also — a form in which the calcareous 



walls are quite transparent — there is a very interesting 

 view of the polypides in the interior of their cells. 



The distribution of the species, geographical and bathy- 

 metrical, finds a prominent place in the Report. An 

 instructive map is appended in which the oceans traversed 

 by the Challenger arc divided into seven regions, three 

 being to the north and four to the south of the equator, 

 each including 90" of longitude. In each of these regions 

 the stations from which any species of Polyzoa were 

 obtained are indicated. 



The bathymetrical range varies within wide limits. 

 The greatest depth which yielded any species to the 

 dredge was 3125 fathoms, in the North Pacific region. 

 From this vast depth four species were procured, and 

 between it and quite shallow water a great number of 

 stations of very various depths are recorded. 



One of the most unexpected facts brought out in the 

 Report is the very wide bathymetrical range enjoyed by 

 certain species. Thus Cribrilina monoceras is one of the 

 four species brought up from 3125 fathoms in the North 

 Pacific, while the same species was obtained from 1325 

 fathoms in the South Pacific, from 69 fathoms in the 

 South Indian or Kerguelen region, from 55 fathoms in 

 the South Atlantic, and from 35 fathoms in the Australian 

 region. 



This striking difference in the depths inhabited by one 

 and the same species is, however, exceptional ; and so is 

 the wide range of geographical distribution which is here 

 presented by a species occurring at great depths. The 

 study of the bathymetrical distribution of the Challenger 

 Polyzoa shows that " the extent of geographical distribu- 

 tion is to a considerable degree correlative with the 

 bathymetrical, the wider geographical distribution being 

 in most instances coincident with the shallower depths." 

 To this law another striking exception is afforded by 

 the beautiful genus Catenicella, a genus very rich in 

 species, which, though from comparatively shallow 

 water, are almost exclusively confined to the Australian 

 region. 



The thirty-six beautiful plates which illustrate the 

 Report are all that could be desired. Clearly and faith- 

 fully drawn, they place in the hands of the zoologist 

 facilities for the determination of the species which, with 

 the descriptions in the letterpress, leave no excuse for 

 erroneous diagnosis. 



Though the Report is confined to the species obtained 

 during the expedition of the Challenger, the number of 

 these is so large, and the descriptions and figures so exact, 

 that the work will possess a classical value, and be found 

 indispensable by every student of the Polyzoa. 



G. J. A. 



