6 THE VOYAGE OF H M.S. CHALLENGER. 



as upon the histology of the Calcarea in general, having been already refuted by the 

 investigations of F. E. Schulze, 1 Metsclmikoff, 2 and Vosmacr. 3 Everybody feels now 

 convinced that all the Calcarea, like other sponges, possess an ectoderm, mesoderm, 

 and endoderm, and that what Hseckel calls " exoderm " is ectoderm and mesoderm 

 together. I would only call attention to one histological peculiarity stated by Hseckel 

 to be present in some Ascones, viz., to the presence in certain varieties of Ascetta 

 primordialis, Ascetta clathrus, Ascaltis canariensis, and Ascaltis lamarckii, of several 

 layers of endodermic cells. 4 Some more precise statements on this point are very 

 desirable. 



There remains a good deal more to be said about the organisation of Sycones and 

 Leucones, particularly of the latter. The pages of the Monograph dedicated to the canal 

 system of the Leucones (Bd. i. pp. 224-237) are among the weakest portions of the whole 

 work. Prof. Hseckel did not succeed in making out the real features of their organisation. 

 This, however, might have been expected. A true and clear exposition of the anatomical 

 structure of the sponge, in its chief modifications, being the merit of Prof. F. E. Schulze 

 exclusively, is an acquisition of the last ten years. Everyone is a son of his time, and now, 

 though we find many of Prof. Hseckel's conceptions to be erroneous, we must not forget 

 that, compared with the very imperfect ideas of Bowerbank, 0. Schmidt, and others upon 

 the same subject, they represented in their time a considerable advance, and rendered 

 subsequent investigations possible. There were, besides, some other causes whose 

 retarding influence must be noticed. I speak of Prof. Hseckel's phylogenetic hypothesis 

 with respect to the derivation of Sycones and Leucones from Ascones : the Sycones by 

 means of strobiloid gemmation, the Leucones by means of the thickening of walls and 

 the ramification of the canals. 5 



I turn firstly to the Sycones. Having adopted the idea that every radial tube of a 

 Sycon represents an Ascon, Hseckel naturally sought after a homologue to the osculum ; 

 he believes it to be found in the large " dermal ostium " on the tubes of Sycetta primitive!,, 

 Sycaltis perforata, and some other species of Sycones 6 and lays stress upon their morpho- 

 1 >gical significance, ascribing to them also an important physiological function. According 

 to him, these conjectural " dermal ostia " are in some cases the only openings through which 

 water runs into the interior of the radial tubes. Such was the way in which Hseckel 

 was led to his statement as to the existence of " intercanalless Sycones," under which 

 category he described the following forms : — Sycetta stauridia, Sycilla cyaihiscus, Sy cilia 

 cylindrus, Sycilla urna, Sycilla chrysalis, Sycyssa huxleyi, Sycaltis glacialis, Sycaltis 

 testipara, Sycaltis ovipara, Sycaltis perforata, Sycortis laevigata, Syculmis synapta, 



1 Ueber d. Bau u. d. EntwickeL v. Sycandra raphamis, Zeitschr.f. iviss. Zooi, Bd. xxv. Suppl. p. 247, 1875 ; Die Meta- 

 morph. v. Sycandra raphamis, ibid., lid. xxxi. p. 290, 1878. 



2 Spongiologische Studien, ibid., Bd. xxxii. p. 349, 1879. 



3 Ueber Leucandra aspera, &c, Tijdschr. d. Ned. Dierh. Vereen., Dl. v. p. 144, 1881. 



4 Kalkschwamme, Bd. i. p. 144. » Loc. cit., p. 340. c Loc. cit., p. 260. 



