I 70 NATURE AND AFFINITIES OF THE SPONGES. 



Deferring for a while the more minute account of the structure and 

 significance of these monad-lined chambers, brief attention may be directed 

 to certain other broad external features superficially recognizable in the 

 various more highly differentiated sponge groups. As previously stated, 

 in every known variety of sponge three essential elements are invariably 

 present, namely, the collar-bearing flagelliferous cells, the mucilaginous 

 cytoblastema, and the amcebiform cytoblasts, to which other accessory or 

 non-essential elements may, or may not, be added. The relative propor- 

 tions in which these three essential elements exist among the several 

 sections of the Spongida, present some important differences. Among the 

 simplest known sponge forms, as represented by the genus Halisarca, in 

 which these elements occur in their pure and simple condition without 

 any addition of spicula, horny fibres, or other skeletal structures, and where, 

 as just stated, the collar-bearing monads exhibit the ampullaceous plan 

 of arrangement, it will be observed, on further reference to the illustration 

 quoted (PL VII. Fig. I.) that this special system by no means occupies 

 a largely predominating portion of the entire sponge body, a very consi- 

 derable part consisting of the common gelatinous matrix or cytoblastema 

 and its enclosed amcebiform cytoblasts. With the majority of the Siliceo- 

 spongiae, including those forms which have, either with or without siliceous 

 spicules, a horny or keratose skeleton, an almost identical predominance of 

 these elements is met with. Among the more characteristic representatives 

 of the Siliceospongiae, however, including the species of Esperia delineated 

 at PI. VII. Fig. 2, a highly characteristic modification is presented. In 

 such as these the cytoblastema is especially remarkable for its extremely 

 thin and pellucid consistence, this being particularly noticeable in the 

 superficial or peripheral region, where it is supported canopy wise, or after 

 the manner of a tent-covering, from the light and efficient scaffold-work 

 furnished by the projecting spicula. A similar type of structure is, as first 

 pointed out by Professor Clark, highly characteristic of the fresh-water genus 

 Spongilla. That combination remaining to be described, in which the pro- 

 portions of the three primary structural elements exhibit a marked diver- 

 gence from those just noticed, is most conspicuously developed in the 

 section of the Calcispongiae, and, with the exception of the Leuconidae, 

 would appear to be essentially characteristic of that group. As already 

 stated, the collared cells in this section are characterized by their diffuse 

 plan of distribution, the entire surface of the internal chambers and passages 

 being more or less completely lined with them. In correlation with this 

 distribution of the collared cells, it is found that the cytoblastema is, as 

 compared with those elements, reduced to its minimum, being indeed super- 

 ficially, as exhibited in the sections of Grantia compressa in Figs. 3 and 4 

 of PI. VII., altogether inconspicuous. 



A closer examination of those special points by which, in accordance 

 with the author's views, the close affinity of the Spongida with the inde- 

 pendent collar-bearing Discostomata is held to be substantiated, may now 



