24 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



neck and filament of the thread-cell are inside the vesicle, which 

 contains a poisonous fluid. If now a small animal touches the 

 cnidocil (trigger-hair), the neck is first turned inside out, everted 

 (d), and then, by means of its spines, becomes fixed in the animal. 

 The thread is now everted within the wound, and the poisonous 

 fluid exerts a paralyzing influence. The mouth of the animal is 

 very extensile, and capable of taking in morsels of considerable 

 size (B/ef). Within the digestive cavity movement of fluids, &c., 

 is mainly effected by the action of the endodermal flagella. The 

 endoderm-cells vary much in size at different times, and can 

 extend themselves so as to almost obliterate the cavity. It is 

 stated that some of these cells are glandular, secreting a fluid 

 which diffuses into the digestive cavity, where it dissolves and 

 breaks down the food; but digestion is also largely infra-cellular, 

 the pseudopodia ingesting solid particles in the same way as in 

 Amoeba. Within these cells the food is reduced to solution, or 

 fine division, and readily passes to the other parts of the body by 

 diffusion. Undigested remnants are passed out through the mouth. 

 In Hydra mridis the presence of chlorophyll probably enables 

 the nutrition to be partly effected as in green plants. 



2. Katabolisin. Secretions are elaborated by glandular cells 

 in various parts of the body. All parts of the body alike give 

 rise to waste-products, which are secreted into the surrounding 

 water, and in the process of Respiration oxygen is taken in 

 (cf. p. 10). 



3. Reproduction is of two kinds, asexual and sexual, the one 

 irrespective of, the other dependent on, the reproductive organs. 



(1) Asexual Reproduction (A and A') is effected by gemmation, 

 and is not limited to any special time of year, but depends upon 

 the food supply and temperature. A small knob grows out from 

 the side of the body, and not only the ectoderm and endoderm but 

 also the digestive cavity of the parent are continued into it. This 

 bud (&) lengthens, develops a mouth and tentacles, and is finally 

 pitched off at its proximal end, which becomes an attachment 

 disc. Several buds in the same or different stages of develop- 

 ment may be present on one individual at the same time, and, 

 under favourable circumstances, these may themselves bear buds 

 before becoming detached. Fission is not known to occur in 

 nature, but a Hydra may be cut into several pieces, each of which 

 can develop into a new and perfect individual. 



