474 EVOLUTION OF ANIMALS Part V 



thelio-muscular cells equip hydra to respond to its environment. The nerve 

 plexus acts as a unit and the impulses appear to travel in either direction over 

 a given process, as in a telephone conversation the speaking goes first one way 

 and then the other on the same wire. In higher animals the incoming and out- 

 going impulses travel on different pathways. There is a concentration of the 

 plexus about the mouth which suggests the more prominent nerve ring around 

 the mouth of a starfish. Investigators have shown that in hydra the processes 

 of different nerve cells may touch but are not continuous. Thus, there is a 

 synapse, a break over which the nerve impulse jumps from one cell to another 

 as in higher animals. 



Formative (or interstitial cells) are small cells wedged in between those of 

 the epidermis and gastrodermis, the lining of the enteron. They behave like 

 embryonic cells, still capable of developing into something different; some of 

 them become sex cells, many become stinging cells (Fig. 23.8). In the human 

 bone marrow, there are embryonic cells that differentiate throughout life into 

 specialized blood cells. 



A stinging cell (cnidoblast) is one that forms within itself the nonliving 

 mechanism called a nematocyst (Fig. 23.8). In the epidermis, mature stinging 

 cells occur close to the outer surface, are numerous on the body, and abundant 

 on the tentacles. Nematocysts are microscopic harpoons expelled from the 

 stinging cells against the hydra's prey and enemies. They are the unique sting- 

 ing mechanism of coelenterates. Each one carries a charge of poison. Those 

 of hydra are harmless except to minute animals, but the stings of larger jelly- 

 fishes and the Portuguese man-of-war are very painful. The fully formed 

 nematocyst is a transparent capsule containing a minute coil usually termed a 

 thread, shown to be a tube in some species and believed to be so in all. Poison 

 is secreted by the stinging cell and is in some way carried by the nematocyst 

 when the latter is discharged. One side of the cell is ordinarily exposed and 

 the triggerlike cnidocil that projects from it is supersensitive to stimulation. 

 Stinging cells respond directly to stimuli. The threads of some nematocysts 

 pierce their prey (Fig. 23. 8C). There are four kinds of stinging cells each of 

 slightly different structure, usually not visible except by special preparation. 



The expulsion of nematocysts is too sudden to be clearly observed. As the 

 tentacle of a living hydra is viewed through the microscope, they can be seen 

 each with a thread coiled within the capsule. When the tentacle is stimulated 

 by pressure or by weak acid, they are instantly expelled and the capsules lie 

 outside the tentacle with their threads uncoiled. It is believed that before ex- 

 pulsion the threadlike tube is inverted in the capsule like a glove-finger pulled 

 inward. When the nematocyst is expelled the tube is rapidly everted by the 

 pressure on the capsule as it is shot out of the cell. 



Stinging cells are wandering cells. Many of them migrate from the epi- 

 dermis, across the mesoglea, go through the gastrodermis into the enteron and 



