366 ORDER FLAGELLATA-EUSTOMATA. 



to that of the preceding less complex organic series. As there made clear, the 

 only substantial distinction found to subsist between the Spongida and the inde- 

 pendent collared Flagellata is manifested by the circumstance that, while in the 

 latter instance the characteristic collared monads are naked, and more or less 

 completely exposed to view, they are in the case of the Spongida associated to- 

 gether and completely concealed within specialized excavations of a common 

 gelatinous matrix, the zoocytium or cytoblastema. Hence the two groups are here 

 accepted as co-ordinate sections of the same primary subdivision of the Protozoa, 

 which, as intimated on a preceding page, may be conveniently distinguished by the 

 respective titles of the Discostomata-Gymnozoida and the Discostomata-Sarcocrypta. 

 So far the social types Phalansterium and Protospongia are the nearest annec- 

 tant forms between these respective groups, though in all probability such small 

 hiatus as yet exists will be still more effectually obliterated by the results of future 

 investigation. 



The further subdivision of the Spongida into minor sections or sub-orders may be 

 most conveniently accomplished with reference to the nature of their skeletal 

 elements, as below. 



Sub-Order I. MYXOSPONGI^E ., .. No accessory skeletal elements. 



,, II. CALCISPONGI^E .. .. Skeletal elements represented by calcareous spicula. 



,, III. SILICOSPONGI^: .. .. Skeletal elements consisting of siliceous spicula. 



IV. KERATOSPONGI^: .. .. Skeletal elements consisting of horny fibre. 



A systematic description of the multitudinous representatives of the Spongida 

 not falling within the scope of the present manual, students desiring to familiarize 

 themselves with their more minute histologic characteristics are referred to the 

 complete works or separate pamphlets of Bowerbank, Carter, Oscar Schmidt, Ernst 

 Haeckel, F. E. Schulze, W. Marshall, and numerous other authorities quoted in the 

 bibliographical list appended to this treatise. In all instances the collared monads, 

 as here described, constitute the one constant and primary factor of the living 

 sponge-stock, the various plans upon which these are grouped together, and more 

 especially the nature and mode of disposition of the skeletal elements, mostly but 

 not universally developed, affording the readiest clue to their generic and specific 

 identification. 



Order VI. FLAGELLATA-EUSTOMATA, S. K. 



Animalcules possessing one or more flagelliform appendages but no 

 locomotive organs in the form of cilia ; a distinct oral aperture or 

 cytostome invariably developed ; multiplying by longitudinal or transverse 

 fission or by the subdivision of a whole or part of the body-substance into 

 sporular elements. 



The number of forms that have to be included in this highest section of the 

 typical Flagellata has been largely increased through the lately published researches of 

 Professor Stein. Previously, the entire group of the Flagellata, including even the 

 simplest monads, had certainly been accredited by Ehrenberg with the possession of 

 a true oral aperture ; his dictum in this connection being accepted by other more 

 recent writers, including Diesing and Pritchard. This attribution to them of so 

 high a structural differentiation was nevertheless, in the majority of instances, purely 

 inferential, being deduced simply from the recognized presence of ingested food- 

 particles within the body-sarcode of the animalcules examined. As demonstrated, 

 however, in this treatise, there exists a very considerable series of forms, scarcely to be 

 distinguished in their broad external characters from the one now about to be intro- 

 duced, in which solid food-particles, while freely ingested, do not obtain access through 

 a specially differentiated oral aperture, but are taken in indifferently at all points of 



