COLOR CHANGES IN ANIMALS — CARLISLE 



423 



perhaps worth remarking that we have examples of both morpho- 

 genetic color change and kinetic color change mediated by the hor- 

 mones of the sexual cycle. 



In the Crustacea (shrimps, prawns, crabs, and wood-lice) the chro- 

 matophores are activated entirely by endocrine means; they are not 

 innervated in any way. In the prawn there are at least 20 different 

 types of chromatophores which respond differentially to 6 or more 

 hormones. As a result a prawn can take on much of the color and 

 pattern of the background, though not perhaps with such precision and 

 speed as the octopus. One feature which many people find surprising 

 is that all the hormones concerned are produced within specialized 

 nerve cells, within the central nervous system. These cells are known 

 as neurosecretory cells (fig. 3) and are grouped in discrete neurosecre- 

 tory centers in various parts of the nervous system. The hormonal 

 material is then conducted inside the nerve fibers to one of two neuro- 

 haemal organs. These consist of enlarged nerve endings grouped in 

 masses against blood spaces and serve as storage and release centers 

 for the hormones produced in the nerve cell bodies which supply them. 

 One of these neurohaemal organs lying close to the eye (fig. 4) is the 

 sinus gland which for long was thought to be the main endocrine gland 

 of prawns. Now we know that it is simply a warehouse and dispatch 

 center for hormones produced in a number of neurosecretory centers 



Figure 3. — Two types of neurosecretory cells from the brain of the prawn. (Magni- 

 fication X 580.) 



