72 • • Harvey B. Holl — On Foml Sponges. 



pjcules. The oscultli- sponges are similarly subdivided, according to 

 form, disposition of the oscules, and presence or absence of an epi- 

 theca. Lastly, the porous sponges are divided into those that are more 

 or less regularly cup-shaped, and those that assume some other form. 



In this arrangement no importance is attached to the nature of the 

 sponge tissue as a character. In fact, M. De Fromen telle states that 

 he considers it to have " a value altogether secondary, for it is not the 

 form or composition of the skeleton which determines the functions, 

 but the futictions themselves which give to the animal the particular 

 form which characterizes it.'' It is not necessary, however, here to 

 discuss this question, further than to observe, that the power which 

 the sponge has to secrete a siliceous spicular framework in one case, or 

 a fibrous rete in another, implies an inherent difference in the nature 

 of the animal, however mueli they may resemble one another in exter- 

 nal character. It will be desirable, therefore, to consider the value of 

 these organs for a moment before proceeding further, and in so doing 

 it will be well. to draw on the living sponge for aid. 



1. Form. As already observed, form, taken alone, is of little value 

 even as a specific character, for whether it be cup-shaped, or tubular, or 

 polymorphous, it is not distinctive, inasmuch as it is common to sponges 

 widely differing in all other characters ; and, indeed, as remarked by 

 Dr. Bowerbank, trusting too implicitly to outward configuration has 

 led to the placing of spicular and fibrous sponges side by side in the 

 same genus. Moreover, the foriii varies greatly at different periods in . 

 the growth of the same individual ; and even in the cup-shaped sponges, 

 commonly the most constant as regards this character, there is a wide 

 difiference in the figure of the old and young individuals of the same 

 species. Hence it is that we know so little of the young condition of 

 many of the fossil sponges, which are not recognized as such, but are 

 regarded, for the most part, as distinct species. 



A circumstance which illusti-ates how the form of the sponge is lia- 

 ble to be governed by accident is mentioned by Dr. Bowerbank in 

 speaking of tlieir reproduction, and is so suggestive that I quote his 

 own words: "On a fragment of a bivalve shell, twenty or thirty 

 sponge gemmules had located themselves, the largest of which did not 

 exceed 3^77 of an inch in diameter, and their distance apart is about 

 equal to their diameter. In their present state," says Dr. Bowerbank, 

 " it is evident that they are separate developments ; and it is equally 

 evident that a slightly further amount of extension would have caused 

 them to merge into one comparatively large flat surface of sponge. 

 We see by this instance that a sponge is not always developed from a 

 single ovum or gemmule, but, on the contrarv, that manv ova or gem- 



