144 



THE RISE OF ANIMAL LIFE 



Fig. 8-8. Hydra somersaulting. 



between these two layers, the ectoderm and 

 the endoderm, is a thin, non-celkilar layer, 

 the mesogloea, which lends support and 

 holds all the cells together. Lying embedded 

 in both the ectoderm and endoderm are the 

 nerve cells, which are connected by minute 

 fibrils to the sensory and muscular cells. 

 They function in coordinating the activity 

 of all the cells. The nerve cells form a net- 

 work, called the nerve net, which connects 

 all parts of the animal, although there is no 

 centrally located mass which could in any 

 way be compared to the brain of higher 

 forms. External stimuli initiate impulses in 

 the sensory cells which are conveyed to 

 the nerve net and through it to the con- 

 tractile fibrils. This is the simplest form of 

 a central nervous system, but it contains the 

 fundamental elements of which all higher 

 nervous systems are built. 



An interesting and intricate part of 

 hydra's response mechanism is the cnido- 

 blast, with its contained nematocyst. 

 Cnidoblasts are located in nests or "batter- 

 ies" along the tentacles, for the most part, 

 although they may occur over the entire 

 body with the exception of the foot, or 

 basal disc, on which the hydra "walks." 

 Nematocysts are derived from the interstitial 

 cells and are usually arranged with one 

 large and several small ones in each bat- 

 tery. There are four different kinds of 

 nematocysts, each having a different use. 

 The largest and most conspicuous type is 

 the penetrating or stinging nematocyst, 

 which upon discharge pierces the body of 

 small crustacean or other aquatic animals 

 that happen to touch the tentacles (Fig. 

 8-7). This nematocyst contains a hollow. 



coiled thread which everts through the 

 trigger-like action of the cnidocil, a slight 

 projecting bristle, when it is touched or 

 stimulated in some other way. It is ejected 

 with such force that it penetrates the soft 

 and even some hard parts of the prey, in- 

 jecting a small amount of poison which has 

 a paralyzing effect on the victim. Once 

 paralysis sets in, the tentacles move in a 

 manner that draws the prey into the mouth 

 and thence into the coelenteron. Other types 

 of nematocysts function in a mechanical 

 rather than chemical manner. When dis- 

 charged, some wrap their threads about a 

 portion of the attacked animal and hold 

 it securely. Others fasten themselves to a 

 portion of the substratum and by contrac- 

 tions of the tentacles make possible a slow, 

 somersaulting type of locomotion (Fig. 

 8-8). 



A second method by which hydra moves 

 from place to place in search of food is to 

 "shuffle" along on its basal disc by means of 

 special cells located in this region. Some 

 species are able to secrete a bubble of air at 

 the basal disc, which carries them to the 

 surface where they float upside down. In 

 this position it is not uncommon for the 

 tentacles to stretch out into thin threads 

 as much as 10 or 12 inches in length. 



Most of the actions of hydra are related 

 to food-getting. A hungry hydra responds 

 readily when a small crustacean or worm 

 comes within reach of its tentacles. If meat 

 juices are placed in the surrounding water, 

 it responds by increased extension and 

 contraction of its entire body. On the other 

 hand, a well-fed hydra responds very little, 

 if at all, to the presence of food. 



