32 THE VOYAGE OF II.M.S. CHALLENGER. 



so frequently repeated in these pages. Of Prof. Hasckel's statements many have proved 

 to be erroneous, but it must not be forgotten that it was his Monograph that called forth 

 and facilitated later investigations, and if we are forced to agree to a certain extent with 

 the judgment of M. Barrois upon the work just mentioned, that " l'imagination y a trop 

 souvenl pris la jdace de l'observation scientifique et froide," everyone will also agree with 

 the other judgment of the same naturalist, that with the appearance of Prof. Hajckel's 

 Monograph "l'histoire des Eponges entra dans une phase nouvelle." 1 



I might now pass on to the description of the forms brought home by the Challenger 

 Expedition. I prefer, however, at this point to summarise the few observations of histo- 

 logical interest which I was able to make during the examination of the Challenger 

 material. I have already spoken of the interesting modifications which the pavement- 

 cells undergo under certain circumstances. The flagellated cells are so sensitive to every 

 kind of preservation that I can state nothing new about their properties. 



There consequently remains only the mesoderm to be discussed. Besides its usual 

 cellular elements, distinguished by Prof. Sckulze " as amoeboid and steUately ramified 

 cells, which I was able to discern in all the Challenger Calcarea, in two cases {Ute 

 argentea and Leuconia multiformis) I observed some very remarkable elements which 

 seem to be intimately connected with the formation of the skeleton. Their form is shown 

 on PL VI. fig. 3c. In comparison with the other elements of the mesoderm, these cells 

 are pretty large, but flatly compressed, and forming with their protoplasm — perhaps under 

 the influence of the preservation in alcohol — a kind of irregular network ; all are provided 

 with a comparatively large oval, or round nucleus. Such cells I found always in twos 

 and threes upon spicules, upon large acerate spicules of Ute argentea. and large triradiato 

 spicules of the parenchyma of Leuconia multiformis. It is known (Metscknikoff 3 ) that 

 calcareous spicules develop in the interior of the mesodermic cells. It may, however, 

 happen — and this is very plausible — that in the cases where the spicula reach a con- 

 siderable size, some other mesodermic cells are also concerned in their growth. 



In addition to the spicules, there are in the sponge generative elements which in 

 their turn are products of the mesoderm. Indubitable young ova — at least in large 

 numbers — I have observed only in two cases : in Leucetta vera and in Pericharax 

 carte ri In Pericharax carteri the ova were of the extraordinary size of 0"3 mm. 

 in diameter, and seemed, like ova of Spongilla (Ganin 4 ), to be surrounded by an 

 endothelium Larva) I found in the following species : — Sycon raphanus, Sycon arcticum, 



i Embryologie, &c, p. 8. 2 Zeitschr. f. wiss. Zool, Bd. xxv. Suppl. p. 253, 1875. 



3 Ibid., Bd. xxxii. p. 370, 1879. * Contributions, &c, p. 14. 



