430 SCIENCE PROGRESS. 



" through coalescence with their fellows attain by degrees 

 the comparatively colossal proportions they present " and 

 then by segmentation produce the larva. The latter he 

 considers not to be a larva in the sense in which the word 

 is used among the Metazoa, but "a motile swarm-gem mule, 

 consisting of a more or less ovate colonial aggregation of 

 typical collared zooids " (p. 183). We shall return later to 

 the question of the larva and its composition. Suffice it for 

 the present to say that Kent draws the conclusion (p. 193) 

 that though sponges form in many ways a transitional group 

 between Protozoa and Metazoa, they nevertheless remain 

 Protozoa In every detail, their position being inseparable 

 from the Choanoflagellata, which in their turn lead back 

 to the simplest flagellate monads. 



II.— THE SPONGES NEITHER PROTOZOA NOR METAZOA. 



If animals composed, as are sponges, of several kinds of 

 tissues, made up of structurally differentiated cells adapted 

 to various functions, are to be classed as Protozoa on the 

 ground that certain of the cells have a great resemblance to 

 certain forms of Protozoa, then the term loses all meaning 

 and becomes impossible to define. We might as well re- 

 gard man as a Protozoan colony, on the ground that his 

 leucocytes resemble amcebse. But that sponges are not 

 Protozoa is no reason for classing them as Metazoa, at least 

 if we use the term Metazoa in a genetic, and not merely a 

 descriptive sense, and mean thereby a group or sub-king- 

 dom descended doubtless from the Protozoa, but through a 

 common ancestor which had advanced beyond the stage of a 

 Protozoan colony. This, the usual conception of the term, 

 renders quite intelligible the idea of the existence of other 

 phyla, which like Metazoa are descended from Protozoa, but 

 along independent lines of descent. Thus some authors 

 who have been unable to entertain the notion that sponges 

 were to be classified as Protozoa, have nevertheless been 

 equally unable to regard them as belonging to the Metazoa. 



Butschli (1883, p. 424) was the first to advocate such an 

 idea, and says : " I am of opinion that the group of sponges 

 is one quite shut off from other Metazoa, which arose quite 



