262 ZOOLOGY 



multiplied. In addition to reproduction bj' means of eggs and 

 the buds already referred to, there are special kinds of buds 

 formed to tide the species over some difficult period such as 

 \vinter or periodic drought. 



The Hydra, which is the type of attached animal we propose 

 to study, belongs to the i)hylum of Cadenterata, — a phjdum 

 that includes both sponges and certain organisms ^^•ith nettle 

 organs. The Ccelenterata are, for the most part, sessile 

 animals. Consequently they have a more or less radial or 

 tree-like form, since animals attached to the earth tend to 

 form a c>'lind]-ical stem with branches springing out on all 

 sides of it. They have also a system of internal cavities 

 serving for the digestion of food as well as for a body-cavity. 

 The l)ody-wall (which is also the wall of the food-cavity) is 

 made up of two layers, — an outer skin, or ectoderm, which is 

 protective and contains sense-organs, and an inner skin, or 

 entoderm, which has a digestive and absorlring function. 



Tlie sponges are sessile ; have no nettling organs, and 

 have the body-wall perforated by man}' fine incurrent open- 

 ings and a large exhalent opening (Fig. 2.53). ]\Iost sponges 

 produce buds and give rise to colonies ; they also frequently 

 produce little round liodies which survive when the rest of the 

 species is killed in the winter and hatch out the following spring. 

 A few sponges are simple indi\nduals, having the form of a 

 cask. The delicate body-wall is supported by a network of 

 spicules made up, in the different groups, of different materials. 

 In some cases the spicules are limy, as in all the common 

 fresh-water species ; in other cases they are silicious, as in the 

 glass sponges ; while still others are horny, like the liath-sponges. 

 Generally the sponge is not a simple individual, but is a colony 

 made up of a number of individuals (zooids) that have budded 



