COELENTERATES IN GENERAL 



149 



177. Behavior. — Coelenterates respond to various stimuli, their 

 responses in general being similar to those of the hydra. The hydra, 

 however, has only scattered sensory cells, while many of the other 

 coelenterates develop specialized sensory structures. On the tentacles of 

 a jellyfish are groups of tactile cells. Between the bases of the tentacles 

 and along the margin of the bell are structures which are believed to 

 function as organs of equilibrium and hence are called statocysts. Other 

 groups of cells are recognized as being olfactory. Finally, there are 

 pigment spots which are sensitive to light. 



178, Reproduction. — In coelenterates generally the same types of 

 reproduction occur as in the hydra. In the case of the hydra, however, 



Ephyroi 



Eggcelip 



Z*^ 2-ceIlsfage 



IfBlasfuIa Gasfrula 



- FertiHi-afion 

 sperm cell from 

 another animal 



Planula 



B C - E F 



Fig. 65. — Diagram illustrating the life history of a scyphozoan jellyfish (Aurelia). A 

 section of the body of a female animal is shown with gonads {A), from one of which an egg 

 cell IS produced. This is fertilized by a sperm cell from another animal, passes through the 

 two-cell and four-cell stages, later becomes a blastula (B), then a gastrula (C), which is 

 shown in section, and finally develops into a ciliated planula larva (D). After a time this 

 becomes attached and changes to a scyphistoma (E), from which is developed a strobila {F) . 

 Each ephyra (G) from this strobila is the young of another animal. 



the one individual exhibits all these types, while in other coelenterates 

 asexual reproduction is often restricted to the polyps and sexual reproduc- 

 tion to the medusae of the same species. Budding may by its repetition 

 give rise to colonies consisting of many hundreds and even thousands of 

 individuals. Medusae remain single, are usually either male or female, 

 and shed germ cells into the water. While, as a rule, they exhibit sexual 

 reproduction, jellyfishes may produce other jellyfishes by budding from 

 the surface of the manubrium. When the sex cells unite in fertihzation, 

 the embryo which develops grows into a ciliated free-swimming larva 

 known as a planula, which, after its free life, settles down and becomes 

 the parent individual of a hydroid colony. 



Among the scyphozoan jellyfishes occurs an interesting type of 

 budding called sfrohilation (Fig. 65). The planula, after becoming 



