152 NATURE AND AFFINITIES OF THE SPONGES. 



science, the latest evidence in this direction, a few months prior only to 

 his much-to-be-lamented decease, was produced by Professor Clark him- 

 self. Already, in ' Silliman's American Journal' for February 1871, he 

 had drawn attention to Cienkowski's new social monad genus PJialaiiste- 

 riiim, and indicated its apparent close affinity to the collared types Codosiga 

 and Salpmgceca discovered by himself, but from which they differed most 

 essentially in the less conspicuous development of the characteristic collar, 

 and in their social or colonial occupation of a common gelatinous matrix. 

 This latter point more especially was cited as indicative of a still more 

 intimate relationship with the sponges than that presented by the two last- 

 named genera. Professor Clark further placed on record in the same serial 

 the results of his recent investigations into the ultimate structure of the 

 American Fresh-water Sponge, Spongilla arachnoides J.-Clk., the results 

 confirming substantially, and adding considerably to, the data elicited through 

 his previous examination of the marine calcareous type LeJicosolcnia botry- 

 oides. In this fresh-water form, however, the characteristic collared flagellate 

 cells were found occupying definite spherical excavated chambers, corre- 

 sponding with those described by Mr. Carter of Spongilla alba under the 

 name of ampullaceous-sacs, which received from Professor Clark the equi- 

 valent title of the " monad chambers." Taken in its entirety, the sponge- 

 body of this species was declared to be composed of the three following 

 distinct and well-marked elements : Firstly, of a common, glairy, gelatinous 

 basis, within which all the remaining constituents were embedded, and 

 which he designated the cytoblastenia. Although presenting the same 

 characteristic consistence throughout, this glairy cytoblastenia exhibited a 

 separation into two distinct regions, the one consisting of a thin superficial 

 stratum, stretched out on the points of the larger externally projecting 

 spicula, after the manner of a tent-canvas extended upon the ends of its 

 supporting poles. Within this peripheral cytoblastematous layer, designated 

 by Professor Clark in its separate form the " investing membrane," there was 

 no trace to be found of the monad-chambers. These latter, which consti- 

 tuted the second and most important structural element, were confined 

 entirely to the deeper and comparatively solid substratum of the cytoblas- 

 tema, and which he therefore distinguished, with relation to its contents, as 

 the " monadigcrous layer." The third and remaining essential element recog- 

 nized by Professor Clark consisted of the innumerable amoebiform cells or 

 bodies scattered more or less abundantly throughout the substance of the 

 cytoblastenia, and most conspicuously visible, in consequence of the absence 

 of the monad-chambers, in the thin superficial stratum or so-called investing 

 membrane. These amoebiform cells were distinguished by Professor Clark 

 merely by the name of the " cell-elements " of the cytoblastema, but, for 

 convenience, may be appropriately designated the " cytoblasts " or 

 " cytodes." Like simple cytodes, they were shown, as also pointed out by 

 Professor Haeckel, to possess no distinct cell-wall, and, unless specially 

 focussed for, were scarcely to be distinguished from the cytoblastenia in 



