NATURE AND AFFINITIES OF THE SPONGES. 165 



submitted at considerable length. With this evidence, however, has now to 

 be amalgamated the results of more recent research, as educed both by the 

 author's personal researches and those of contemporaneous investigators. 



Commencing with the general structural features of an adult sponge, 

 as met with throughout the several more important groups into which 

 the class Spongida is most naturally divided, it will be found that in 

 any one of them, the three elements, demonstrated through the inves- 

 tigations of Professor Clark, constitute the actual and essential com- 

 ponents of the sponge-body. These three, the collar-bearing flagellate 

 monads, the hyaline and mucous-like cytoblastema, and the included 

 amceba-like cytoblasts, will be invariably found in every sponge, more 

 generally with superadded skeletal structures, and often with a greater or 

 less proportional preponderance or reduction of the first, second, or third of 

 the essential elements enumerated, but in no instance presenting an entire 

 absence of any one of them. Out of these three elements, again, it is 

 beyond question that the first-named, or collar-bearing flagellate monads, 

 lay claim to the foremost position in the sponge economy, and that the two 

 remaining ones are, as compared with them, entirely subsidiary. Separating 

 the collar-bearing units from those two subordinate elements, as shown 

 more especially at PI. VIII., and comparing them with the independent 

 collar-bearing flagellate monads, figured in Pis. II. to VI., the close 

 identity of the two cannot possibly fail to be recognized. This likeness, 

 furthermore, is found to be not merely general and superficial, but to extend 

 to every point that may be enumerated. Neither is such likeness common 

 and unimportant, or comparable to such as is usually found between 

 ordinary tissue-cells, or to more practically illustrate the case, to such 

 as exists between an amoeba and a leucocyte or white blood-corpuscle. 

 Amoebiform cells, indistinguishable in their isolated condition from ordi- 

 nary amoebae, recur again and again as the constituent or associated 

 elements of the organic tissues of both vertebrate and invertebrate 

 animals. That special modification of a simple cell, however, exemplified 

 by the independent collared monads or Choano-Flagellata, and precisely 

 reflected in the essential elements of the sponge-body, finds its counter- 

 part nowhere else throughout the entire range of organic nature. While 

 these two can be correlated and shown to harmonize with each other in 

 every detail, it becomes self-evident that all attempts to co-ordinate either 

 of them with any other structures are rendered nugatory, and are, in point 

 of fact, attempts to compare that which is altogether uncomparable. In 

 what manner the collar-bearing monads diff"er essentially from all other 

 known unicellular structures, is explained at length in the section devoted 

 to the systematic description of their order. Briefly, however, it may now 

 be stated that the all-important distinction here insisted on is connected 

 with that peculiar structure the " collar " and its accompanying functions. As 

 demonstrated by the author, this "collar" is not a mere funnel-shaped 

 expansion of inert sarcode, as might be inferred from the earliest accounts 



