210 NATURAL HISTORY OF BHOMPSTON POND. 



Hydra viridis (plate 2, fig. 31) is the only example of the 

 Ccelenterata which seems to be present in the pond. Its 

 sac-like form is attached (frequently to the root of Duckweed) 

 by an adhesive foot (a) ; the other end, which terminates in an 

 oral cone (b) and tentacles sways freely in the water. Usually 

 it is stretched into a most elongated form, but on stimulation 

 is able to suddenly contract (c). The animal feeds on water- 

 fleas and allied crustaceans, captured by means of its stinging 

 tentacles (fig. 32). If it is killed with corrosive sublimate 

 before it has even had time to contract, then fixed, stained, 

 and cast in wax, a sufficiently thin section (about l-1000thin. 

 in thickness) may be cut, which will display the minute 

 structure of the body wall (fig. 33, a), enclosing a single space, 

 the ccelenteron (b) open to the exterior only at the mouth. 

 The ectoderm (fig. 34, a), a thin single layer of epithelial cells, 

 is separated from the endoderm (b) by the structureless 

 mesoglcea (c), in which are fixed both the longitudinally 

 arranged muscular tails of the ectoderm cells, and those of the 

 endoderm cells placed transversely. The shortening and 

 lengthening of the animal is due to the action of this 

 musculature. Near the mesoglcea, closely packed in between 

 the bases of the ectoderm cells, are the interstitial cells (d), 

 whose function seems threefold. Some may accumulate to 

 form the reproductive organs ; or to pass to the exterior and 

 become cnidoblasts (stinging cells) (e), each of which contains 

 a nematocyst (thread cell) (f) and a cnidocil (g), the simplest 

 known sense-hair which transmits a stimulus, bringing about 

 the expulsion of the thread (fig. 35 shows an exploded 

 nematocyst which the writer found adhering to an attacked 

 nauplius), or finally they may pass outwards and develop into 

 ordinary ectoderm cells. The endoderm cells are large, and 

 contain a number of symbiotic algae (fig. 34, h), giving the 

 animal its green colour. Some of these cells are produced 

 into pseudopodia (i) or flagella, while others are granular and 

 secrete a digestive fluid. The parts of the prey not digested 

 in the coelenteron are bodily ingested by the pseudopodia, 

 and digested in food vacuoles in the endoderm cells, the 



