BIONOMIC CHARACTERS OF PORIFERA 1009 



ous or siliceous skeleton — either continuous or consisting of separate 

 spicules — are capable of preservation in a fossil state. The vertical 

 distribution of marine species ranges from the shore zone down 

 to the greater depths of the sea. Not infrequently species are found 

 which regularly undergo an exposure of several hours between 

 tides, though most littoral species occur below low-water mark, or in 

 tide pools from which the water is never drained. Sponges will 

 grow wherever a suitable surface for attachment is found, the most 

 usual substratum chosen being cliffs, boulders, shells, or the stems 

 and "roots" of the larger algse. In deeper and quieter water, 

 the sandy and gravelly bottoms are inhabited by sponges, and in 

 the great depths they occur on the oozes and other soft deposits. 

 A pseudovagrant (epi-vagrant) benthonic habit is assumed by a 

 number of species, which attach themselves to the carapaces of 

 Crustacea. Certain sponges bore into shells and other calcareous 

 substances, forming extensive galleries and commonly destroying 

 the shell. Clione sulphurea, common on our Atlantic coast, com- 

 pletely riddles shells, and then forms large irregularly rounded 

 masses of a sulphur yellow color, often entirely enveloping the 

 shell. 



The reproduction of the sponges is either asexual or sexual. 

 In the former case, buds are formed, which, growing larger, with- 

 out detaching themselves, put out buds of their own, thus forming 

 a colonial aggregation. Sponges torn into several pieces will fre- 

 quently form as many new individuals, and sponges which were 

 placed in close juxtaposition by Bowerbank in a relatively short 

 time united into one. A method of internal gemmation occurs, in 

 which groups of cells, or gemmulse, become detached and after a 

 time develop into complete sponges. Sexual reproduction, from 

 either hermaphrodite or sexually distinct parents, leads to a free 

 swimming blastula. This develops into a gastrula, which attaches 

 itself and develops into the adult. Thus a mero-planktonic stage 

 occurs in sponges, which serves as a means of extensive distribu- 

 tion. 



III. CCELENTERATA. 



Hydrozoa. The Hydrozoa are typically marine Coelenterates, 

 though a few species occur in fresh water, c. g., H\dra firidis and 

 H. fusca, Cordilophora lacustris and the fresh water medusa of 

 Lake Tanganyika : Limnocuida tanganyikcc. Some Scyphomedus?c 

 (Aurelia, Cyanea), according to Moseley, seem to prefer to float 

 near the mouths of fresh water streams ; while in New South Wales 



