NATURE AND AFFINITIES OF THE SPONGES. 153 



which they were embedded. Seen under the most favourable auspices, their 

 substance was demonstrated to be slightly granular, and that they con- 

 tained a subcentral, spherical, and more highly refractive nucleus or 

 endoplast. The contours presented by these cytoblastic bodies varied 

 greatly, ranging from simply ovate to every variety of irregular and jagged 

 outline, and their periphery often taking the form of more or less prolonged 

 caudiform projections, directly comparable with the pseudopodic appen- 

 dages of a typical Amoeba. In addition to these three essential elements, 

 e. g. the transparent structureless " cytoblastema," the " collared monads," 

 and the polymorphic amcebiform " cytoblasts," one conspicuous but non- 

 essential structural element, as represented by the supporting or strength- 

 ening siliceous spicula, remains to be mentioned. The spicula in the 

 sponge in question were of two sorts, large and small, and were found 

 in either case to be confined exclusively to the cytoblastematous layer, and 

 not to intrude into the monad-chambers ; while the larger ones penetrated 

 into the deeper substratum of this element, the smaller ones occurred only 

 in the more attenuate peripheral region, and were evidently built up or 

 secreted by the cytoblastema or its enclosed amcebiform cytoblasts. The 

 exclusion here made of the secreted spicula from the category of essential 

 elements, while a departure from the course taken by Professor Clark, is 

 justified by the fact that sponges exist e. g. the Myxospongiae, including 

 Halisarca and its allies in which, while all the other three elements 

 are fully represented, spicula or skeletal structures of any kind are entirely 

 absent. 



Among the data of importance recorded by Professor Clark concerning the 

 organization of the separate collar-bearing elements of the monad-chambers 

 or ampullaceous sacs, has to be particularly mentioned that he demonstrated 

 in this type most definitely the possession by each monad of two or more 

 conspicuously developed and evenly pulsating contractile vesicles. The 

 location of these vesicles was found to be more towards the posterior 

 extremity of the body, their systole and diastole being further described as 

 on the whole extremely slow, but very distinct, if sufficient patience was 

 used to watch them fixedly and without interruption. The last third 

 portion of the act of systole differed in being considerably more abrupt, 

 the vesicle appearing only at such time to contract suddenly. This latter 

 circumstance, taken together with the constant position of the vesicles, is 

 cited by Professor Clark as sufficing to rebut the inference that might 

 otherwise be arrived at, and as actually insisted on by Professor Haeckel, 

 that these vesicles were simply irregular protoplasmic vacuoles, such as 

 occur among undoubted protophytes and various ordinary tissue-cells. 

 Comparing the arguments adduced by Professor Haeckel, in favour of the 

 Ccelenterate affinities of the sponges, with the actual structural composition 

 of Spongilla and Leucosolcnia, elicited by his own special investigations, 

 Professor Clark finally arrived at the decision that the attempted parallel- 

 ism between the two groups must utterly fail, the relationship of the 



