NATURE AND AFFINITIES OF THE SPONGES. 167 



is presented. The component sarcode of the monad's body in this latter 

 case is apparently of thinner or more fluid consistence, permitting of rapid 

 and protean changes of form on the slightest irritation or other stimulus. 

 Thus, when a dissection is made of any living sponge, such as Grantia 

 comprcssa, it is found requisite to examine rapidly with the microscope 

 in order to witness the constituent monads with their collars and flagella 

 in the normal condition of extension ; otherwise, within the space of a few 

 minutes, these organs are, one or both, more or less completely retracted, 

 and their former possessors creeping about the field, or remaining congre- 

 gated in clusters under conditions that render them indistinguishable from 

 ordinary Flagellate monads, or simple amoebae. In this latter instance, 

 they are, indeed, identical in all respects with the amcebiform zooids or 

 cytoblasts scattered throughout the common gelatinous central matrix or 

 cytoblastema of the sponge-body. The more conspicuous modifications 

 of form assumed at will by the collared sponge-monads, either with the 

 collar and flagellum extended with the former organ alone, or with both 

 of these two retracted, will be found abundantly illustrated in Figs. 2-38 

 of PI. VIIL, devoted especially to the histology of the Spongida. Among 

 them two or three examples occur which deserve particular notice. Thus, 

 at Fig. 17 of the plate quoted, is represented an individual monad w^hich, 

 after the retraction of the collar and flagellum, has thrown out innumer- 

 able long slender pseudopodia, which convey to it an appearance highly 

 suggestive of that of the Radiolarian type Actinophrys. Other examples, 

 exhibiting in a less degree the same plan of metamorphosis, are also 

 delineated in connection with the groups or isolated examples numbered 

 respectively 12, 14, 15, 16, 20, and 24. In Figs. 29, 30, 31, a modification in 

 an entirely different direction is shown. Here, with the collars and flagella 

 entirely withdrawn, as before, slender pseudopodia are emitted from the peri- 

 phery, terminating in distinctly capitate extremities which recall to mind the 

 specialized suctorial tentacles of the Acinetidae. Recovering from the disturb- 

 ing influence which has brought about any of the various metamorphoses 

 above enumerated, the emitted pseudopodia or lobate sarcode extensions 

 are, after a while, drawn in, and the normal form with the extended collar 

 and flagellum again assumed. At Fig. 18 of PI. VIII. will be found 

 examples of such sponge-monads, which after a short tenure of a vagrant 

 amoeboid condition, have reattached themselves to a minute spiculum of the 

 parent sponge, and resumed the customary aspect of the typical Spongozoa. 

 Were it not explained that these readherent collared monads belonged 

 to a sponge-stock, it would be impossible to distinguish them from the 

 representatives of such independent collared animalcules as Monosiga 

 Steinii, represented at PL IV. Fig. 12. The collared sponge-monads, thus 

 reattached, soon throw out around them a thin investing film of hyaline 

 cytoblastema, as shown at Figs. 19, 21, and 22, and are thus capable, 

 without any other extraneous assistance, of either repairing a mutilated 

 sponge-stock, or of building up an entirely new one. With reference to 



