COLOURS 59 



and release of hormones. The sinus gland receives its name because 

 it lies alongside a small blood sinus. It is doubtful whether it 

 actually secretes hormones itself, but there is no doubt that large 

 quantities of hormones are found in it. These hormones pass along 

 fibres leading from cells located in the brain and in the X-organ 

 in the medulla terminalis (M.T. X organ). In some species there 

 are additional fibres leading to the sinus gland which seem to come 

 from other parts of the nervous system. The sinus gland is probably 

 best regarded as a reservoir and releasing point of hormones. 



A similar release point is found in the region just behind the 

 post-oesophageal commissure. Two fine nerves, the post-commissure 

 nerves, leave the commissure and end in flat plates which are partly 

 joined to the wall of a blood sinus. This region of the nervous 

 system is particularly rich in hormones which control colour 

 change. The details of these substances are complicated and not 

 yet fully worked out; a review and introduction to the vast literature 

 is given by Knowles and Carlisle (1956). The situation is com- 

 plicated further by the fact that crabs and prawns differ in details 

 of chromatophore control. Substances have been extracted from 

 the eyestalks and post-oesophageal commissures of prawns which 

 cause the chromatophores to contract their pigments, and a melano- 

 phore expanding substance has been extracted from the sub- 

 oesophageal ganglion of the crab Uca. In this latter creature there 

 is evidence that six hormones may be involved in its colour control; 

 expanding and contracting substances are produced for the red, 

 white and black chromatophores. However, it must not be thought 

 that the hormones are always specific for chromatophores of a 

 particular colour. In Leander one substance is capable of causing 

 contraction of the red pigment in the white-red-yellow chromato- 

 phores, and simultaneously causing expansion of red pigment in 

 the red-yellow chromatophores. 



Further studies on Uca have shown that it has a rhythm of 

 colour change that is remarkably persistent. The crab is pale at 

 night and dark during the day, and it will maintain the change 

 from pale to dark and back again in a regular rhythm even when 

 kept in constant darkness at a constant temperature for as long as 

 two months. A curious additional feature is that the time of 

 maximum darkening shows a rhythm which is tuned to the tides. 

 The time of greatest expansion of the melanophores becomes about 

 fifty minutes later with each successive day, and the time coincides 

 with a period of about an hour before low tide. The daytime 



