20 THE VOYAGE OF H.M.S CHALLENGER. 



carted, n. sp., or in Leucetta hcecMiana (PI. VIII. fig. 1), these quadriradiate spicules 

 being present in the exhalent and absent in the inhalent canals, both the systems of 

 the canals can be readily distinguished from one another, and I am inclined to see in this 

 fact an anatomical confirmation of the conceptions of Prof. F. E. Schulze, 1 according to. 

 whom the pavement-cells of the inhalent canal system are of ectodermic, those of the 

 exhalent of endodermic, origin. 



I have now to make a summary of my conclusions ; I formulate them as follows : — 

 The Leucones are nothing but modified Sycones with non-articulated tubar skeleton ; 

 their flagellated chambers are complete homologues of the radial tubes ; their exhalent 

 canals owe their origin to the invaginations of the inner cavity, and their inhalent 

 canals are to be regarded as homologous with the intercanals of the Sycones. 



r There are no further complications of the canal system in the group Leucones which 

 require particular explanation ; and now — since I have completed its definition, including 

 its development, and since some corrections concerning Hseckel's statements as to the canal 

 system of the Sycones have been made, as there is nothing to be added to Prof. Hseckel's 

 description of this system in the Ascones — I can return to the important question put 

 some pages before, whether the proj)erties of the canal system can be really used as 

 characters for the definition of families. It is quite evident that this question must be 

 answered in the affirmative. For it is the canal system which is, in Calcarea, the princi- 

 pal vital organ, and it is the type upon which the mutual disposition of the component 

 parts of the second important organ, the skeleton, as well as the greater or smaller 

 development of the connective tissue, depends. And there are in the Calcarea, beside the 

 form and the quality of the spicules and the external form of the animals, both of which 

 are very variable, no other characters of systematic value. Most of them — some Ascones 

 (Keller, 2 Barrois 3 ), and very probably all Sycones and Leucones — have the same kind of 

 development, characterised by the well-known Amphiblastula ; and although the species 

 Ascetta blanca, Ascetta primordiaUs, Ascetta clathrus, and probably some other Ascones, 

 have a larva of a different type, this difference, even in the eyes of a professional 

 embryologist, is of no greater value than to prompt him to the following remark : — 

 " Wenn dies (development of some species of Ascandra like that of Sycandra and 

 Leucandra,) sich durch unmittelbare Beobachtung bestatigen sollte, so wiirde das nur 

 zeigen, dass zwischen der Gattung Leucandra und Sycandra eine nahere Verwandschaft 

 als zwischen Ascandra und Ascetta besteht " ; 4 not that Leucandra, Sycandra, and 

 Ascandra ought to be put together in order to oppose them systematically to Ascetta ; 

 and even this opinion, the embryological facts being of a very precarious nature, can still 



1 Die Plakiniden, Zeitsehr.f. wiss. Zool, Bd. xxxiv. p. 437, 1880. 



2 Ueber d. Anat. u. Entwick. einiger Spongien, 1876, p. 32. 3 Embryologie, &c, p. 35. 



4 Even if thi9 (development, &c.) should be confirmed by direct observation, it would merely show that 

 between the genera Leucandra and Sycandra there is a nearer relationship than between Ascandra and Ascetta. 

 E. Metschnikoff, Zeitschr. f. wiss. Zool, Bd. xxxii. p. 370, 1879. 



