234 THE AMERICAN MUSEUM JOURNAL 



sponge. This shows itself particularly in the movements of the 

 osculum and -pores. 



Sponges may grow together if placed in contact, or, on the 

 other hand, fragments cut from a sponge can be made to live 

 and grow separately. This peculiarity is utilized in connection 

 with the artificial propagation of the commercial sponges. 

 Sponges do not, however, regenerate parts which have been 

 cut off, although the original sponge may go on growing as if 

 nothing had happened. 



POSITION OF SPONGES IN THE ANIMAL KINGDOM. 



The relation of sponges to other animal forms has always 

 been very tmcertain. The choanocytes of the endoderm seem 

 to connect them with a group of colonial Protozoa known as 

 Choanojiagellates. These are the only other animal forms 

 which have "collared cells." In fact certain colonies of Choan- 

 oflagellates (Proterospongia) very much resemble primitive 

 sponges. On the other hand sponges have often been grouped 

 with the Coelenterates, on account of the resemblance of the 

 planula and gastrula larval stages to those of the Jellyfishes ; 

 because of the fixed condition of the adult, the simple structure 

 and the sac-like internal cavity; as well as the supposed re- 

 semblance of the osculum to the Coelenterate mouth. The latter 

 resemblance is only apparent, however, as the osculum does not 

 fiinction as a mouth, nor does it have the same embryological 

 history. Sponges, moreover, differ widely from Coelenterates in 

 their lack of tentacles and "sting-cells," or nematocysts, and 

 are peculiar in having pores, "collared cells," and spicular 

 skeletons. These differences are so important that it has been 

 necessary to recognize the sponges as a separate subkingdom, 

 most probably having a common ancestry in some group imme- 

 diately derived from the Protozoa. 



CLASSIFICATION. 



On account of the difficulties besetting sponge classification 

 many very widely differing schemes have been proposed. The 

 earliest were based largely on external forms and the chemical 



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