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NATURE AND AFFINITIES OF THE SPONGES. 153which they were embedded.Seen under the most favourable auspices, theirsubstance was demonstrated to be slightly granular, and that they containeda subcentral, spherical, and more highly refractive nucleus orendoplast. The contours presented by these cytoblastic bodies variedgreatly, ranging from simply ovate to every variety of irregular and jaggedoutline, and their periphery often taking the form of more or less prolongedcaudiform projections, directly comparable with the pseudopodic appendagesof 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 nonessentialstructural element, as represented by the supporting or strengtheningsiliceous spicula, remains to be mentioned. The spicula in thesponge in question were of two sorts, large and small, and were foundin either case to be confined exclusively to the cytoblastematous layer, andnot to intrude into the monad-chambers ;while the larger ones penetratedinto the deeper substratum of this element, the smaller ones occurred onlyin the more attenuate peripheral region, and were evidently built up orsecreted by the cytoblastema or its enclosed amcebiform cytoblasts. Theexclusion here made of the secreted spicula from the category of essentialelements, while a departure from the coursetaken by Professor Clark, isjustified by the fact that sponges exist e. g. the Myxospongiae, includingHalisarca and its allies in which, while all the other three elementsare fully represented, spicula or skeletal structures of any kind are entirelyabsent.Among the data of importance recorded by Professor Clark concerning theorganization of the separate collar-bearing elements of the monad-chambersor ampullaceous sacs, has to be particularly mentioned that he demonstratedin this type most definitely the possession by each monad of two or moreconspicuously developed and evenly pulsating contractile vesicles. Thelocation of these vesicles was found to be more towards the posteriorextremity of the body, their systole and diastole being further described ason the whole extremely slow, but very distinct, if sufficient patience wasused to watch them fixedly and without interruption. The last thirdportion of the act of systole differed in being considerably more abrupt,the vesicle appearing only at such time to contract suddenly. This lattercircumstance, taken together with the constant position of the vesicles, iscited by Professor Clark as sufficing to rebut the inference that mightotherwise be arrived at, and as actually insisted on by Professor Haeckel,that these vesicles were simply irregular protoplasmic vacuoles, such asoccur among undoubted protophytes and various ordinary tissue-cells.Comparing the arguments adduced by Professor Haeckel, in favour of theCcelenterate affinities of the sponges, with the actual structural compositionof Spongilla and Leucosolcnia, elicited by his own special investigations,Professor Clark finallyarrived at the decision that the attempted parallelismbetween the two groups must utterly fail, the relationship of the