OF PROTOZOA. 41 



in the various functions which they perform, but in the actual 

 differentiation of the various parts of the body. The common 

 sponges of commerce, as they come to us for our daily use, 

 would not help us much to understand their nature ; it is only 

 after an attentive study of their living forms, in their native ele- 

 ment, water, that one may comprehend the character of those 

 parts of the dried sponge which are not destroyed by the process 

 of preparation to which they are subjected, before they are 

 brought into market. It is a very easy matter to obtain living 

 sponges, because they abound on our rocky coasts, and in our 

 fresh waters. The sponges of commerce, of which there are 

 apparently several species, are not native ; nor can any of those 

 which grow about us be used for the purposes to which the 

 foreign sponges are adapted ; but yet as far as their nature and 

 general structure are concerned, the native ones are fully ade- 

 quate to illustrate the group. 



The sponge which I have pic- 

 tured here (fig. 21) is a com- 

 mon occupant of our ponds and 

 streams, most frequently adher- 

 ing to and growing on the stems 

 of aquatic plants, or forming low Fig. 21. 



prominences on the surface of stones. It is readily detected by 

 a whitish brown color and bristly surface, and may be found 

 from an almost invisible size to the dimensions of the fist, and in 

 shape either perfectly globular, or oval, or elongated, so as to 

 assume the shape of the stem on which it grows ; or, as I have 

 said, as a low prominence, like a bristling wart, on the stones in 

 brooks, or at the side of the pond. It is only when placed 

 in a glass jar and held up to the light, that, with the help of the 

 microscope, its peculiarities are revealed. After the specimen 

 under observation has recovered from the shock of the removal 

 and expanded again, the first thing that strikes the eye is a more 



Fig. 21. Slpliydora echinodes, nov. gen. et sp. 10 diam. A fresh-water 

 sponge, s, the emptying conduit ; p, the superfi< ial interstices, forming a part 

 of the anastomozing channels, and the points of ingress for the water. Original. 







