339 



are inhabited by sponges, and in the great depths they 



occur on the oozes and other soft deposits. A pseudo- 

 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 sub- 

 stances, forming extensive galleries and commonly destroy- 

 ing the shell. Clione sulphurea, common on our Atlantic 

 coast, completely riddles shells, and then forms large irreg- 

 ularly rounded masses of a sulphur yellow color, often 

 entirely enveloping the shell. 



The reproduction of the sponges is either asexual or sex- 

 ual. In the former case buds are formed, which, growing 

 larger, without detaching themselves put out buds of their 

 own, thus forming a colonial aggregation. Sponges torn 

 into several pieces will frequently form as many new individ- 

 uals, 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 gemmuhe, 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 distribution. 



Hydrozoa. — The Hydrozoa are typically marine Ccelente- 

 rates, though a few species occur in fresh water.* Some 

 Scyphomedusa? (Aurelia, Cyanea), according to Moseley, seem 

 to prefer to float near the mouths of fresh-water streams; 

 while in New South Wales these medusae were observed 

 swimming in shoals where the water was pure enough to 

 be drinkable. The majority of species have a sedentary 

 benthonic stage, the hydriform stage, which is generally 



* To the three fresh-water species of hydroids — Hydra viridis, H. fusca (vulgaris), and 

 Cordilophora lacustris — none of which possess a medusiforni stage, has recently been 

 added the remarkable fresh-water medusa of Lake Tanganyika : Limnocnida tangan- 

 yikce Gunther, which, together with a peculiar molluscan and fish fauna, seems to indicate 

 that Lake Tanganyika is a " cut-off " from the Mesozoic sea, with a fauna which has grad- 

 ually become adapted to lacustrine conditions. 



