PROF. DELAGE ON SPONGES. 57 



Undoubtedly, their place is above the Protozoa, among the Metazoa, 

 under the Ccelenterata. The difficulty is to say if they are to be 

 placed as a class at the foot of the phylum Ccelenterata or are to 

 be raised to the dignity of a phylum ; and whether, from a phylo- 

 genetic point of view, they have taken their origin from the 

 Coelenterate branch of the genealogical tree, or if they arose inde- 

 pendently from its stem, forming, as well as the Ccelenterata, a 

 main branch of it. 



As we know nothing of this hypothetical tree, it is better to 

 set it aside for the moment and to discuss only the taxonomic 

 problem. 



Here, the difficulty is that we want a criterion of the value 

 of taxonomic characters. We lack means to establish a relation 

 between the importance of the zoological characters and that of the 

 taxonomic groups. 



That is not a difficulty peculiar to the Spongida. It is the same 

 throughout the animal kingdom. VVe are always at a loss to decide 

 if a given character is fit to determine a phylum or a class, a class 

 or an order, an order or a family, a family or a genus, to say nothing 

 of the intermediate groups. 



Were it a matter of fact, that more accurate observations would 

 afford decisive arguments, we could hope to settle the difficulty. 

 But, it is not so. We all agree as to what are the differences and 

 the likenesses between the Spongida and the Ccelenterata. But, to 

 decide if these differences oblige us to separate them in different 

 phylums, or if these likenesses permit us to put them together 

 in the same phylum is a matter of taste. For instance, the 

 Spongida have pores, have a true mesoderm, but want tentacles and 

 nematoblasts ; the Ccelenterata have no pores, nor true mesoderm, 

 but possess tentacles and nematoblasts. All agree to recognise those 

 characters. But are they characters of classes or of phylums? 

 Who can say ? Where is the touch-stone .-' We could discuss the 

 subject during all the week without coming to a conclusion. The 

 cause is that taxonomy is a subjective matter ; and it is as easy to 

 settle a subjective problem by means of objective arguments as 

 to open a hole in water by throwing stones in it. 



Moreover, />ractzca//y, there maybe means to solve the difficulty. 



If the adequate value of the differences fit to characterize the 

 various taxonomical groups is impossible to fix with precision, yet, 

 all zoologists agree to admit that there is an average value of the 

 importance of characters, required to determine a given group ; 

 and when this importance is evidently far beyond what is necessary, 

 there is no doubt that it is sufficient. For instance, nobody could 

 think of putting in the same phylum two groups whose important 

 organs as the nervous system or the excretory apparatus are built 

 on quite different patterns. 



So the problem may be reduced to this : 



A re there to be fomid betzveeu the Spongida and the Cceletitej'ata 



