16 THE VOYAGE OF H.M.S. CHALLENGER. 



Now, so fur as concerns the histological identity of both these kinds of elements, 

 this identity finds expression even in the modifications which the pavement-cells undergo 

 under certain circumstances. Usually very thin, and provided with fine-grained protoplasm, 

 the pavement-cells appear sometimes far thicker and their granules much larger and more 

 numerous. I have often observed such modifications, and nowhere better than in 

 Leucetta vent. n. sp. (PL VIII. fig. 8). Sometimes these coarse-grained cells form the 

 epithelium either of the exhalent or of the inhalent canals ; in most cases, however, both 

 that of the exhalent and that of the inhalent canals indiscriminately. Since I always 

 found in the same sponge surfaces covered "with common pavement-cells, in addition to 

 the surfaces covered with coarse-grained cells, the supposition that the large granules 

 present a constant constituent part of the cells, characteristic of the species, is not admis- 

 sible. On the other hand, the character of the modification in the cells in cmestion 

 jjroves that it stands in immediate connection with the nutritive process, and there being, 

 as I remarked before, no distinction in this respect between ecto- and endodermic pave- 

 ment-cells, I see in this a new reason for ascribing the nutritive function to the pavement- 

 cells both of endo- and ectodermic origin. I must add that the expression " nutritive 

 function " has been used by me with the meaning " reception of the food," and not inclusive 

 of digestion, all cellular elements of the sponge-organism being probably capable of this 

 latter function. A corollary of the highest importance deduced from the foregoing remarks 

 has already found its application. 



Let us now turn from the Sycones to the phylogenetic affinities of the Leucones. There 

 are amongst the Sycones described by Prof. Hgeckel a good number of forms which, com- 

 pared with Sycetta primitiva, Sycandra raphamts, &c, must be regarded as consider- 

 ably modified Sycones, the differences pervading the whole plan of their organisation. I 

 speak of the type represented in the Monograph by the species Sycetta stauridia, Sycilla 

 urna, Sycilla chrysalis, Sycilla cylindrus, Sycilla cyathiscus, Sycaltis glacialis, Sycaltis 

 testipara, Sycaltis ovipara, and Sycidmis synapta. Every impartial investigator, when 

 comparing, for instance, Sycandra raphanus with Sycilla chrysalis, would refer them 

 to two distinct genera. " In die bis jetzt bekannten Sycortisformen passt Sycortis 

 sycillo'ides nicht gut herein," writes Schuffner, describing a calcareous Sponge of the 

 type of Sycilla chrysalis, " es schliesst sich dieselbe in ihrer Zusammensetzung und in 

 dem Bau des Skelets der Radialtuben vielmehr an eine Sycilla oder Sycaltis-form an.' ;1 

 This is my opinion too. 



Prof. Hseckel, constant to his principles of classification by the spicules, assigns no 

 systematic significance to these differences of organisation, yet they did not escape him. 

 So he distinguished 2 two types of radial tubes, characterising them by differences in the 



1 " Sycortis sycilloules does not seem to be appropriately placed along with the hitherto known forms of Sycortis. In 

 its composition and inthe structure of the skeleton of its radial tubes it is more nearly allied to Sycilla or Sycaltis." 

 Beschreibuiig einiger neuer Kalkschwamme, Jenaitche Zeitschr., Bd. xi. p. 422, 1877. 



2 Kalkschwamme, Bd. i. p. 319. 



