EFFECTS OF PARASITES. 445 



ment, the day pattern of the adult prawn does not vary in 

 distribution. But in the adolescent stage a permanent change 

 in the pattern of individuals with a coat of lines or bars may 

 be induced by appropriately altering their surroundings, leading 

 them to assume, e.g. a uniform coat by extension of the superficial 

 pigment-tracts over the body, and this becomes the permanent 

 adult coat. In this result the character of the stimulus received 

 by the eye is an essential factor in the change, i.e., in the resulting 

 increase in the number of the chromatophores. 



When Hippolyte varians is against a dark background, there 

 is, as stated above, an alternate expansion and contraction of 

 the pigment of the chromatophores in response to day and 

 night. If these prawns are left in the dark, Keeble and Gamble 

 have found that, for some 70-80 hours, an expansion of pigment 

 occurs during the recurring diurnal periods though gradually 

 lessening in intensity. Thus the rhythmic expansion and con- 

 traction of the pigment set up in response to alternating day 

 and night persists after the conditions of illumination have been 

 rendered uniformly nocturnal. 



The conclusion as to the effect of background in modifying 

 the distribution of the pigment in the Crustacea presents a 

 parallel with some of Poulton's results on the development of 

 colour in the larvae and pupae of Insects.* For a fuller 

 statement of the highly interesting and suggestive, results 

 obtained, the reader is referred to the original memoirs. 



Modification of the reproductive system produced by parasites. 

 Giard has drawn attention to the remarkable effects produced 

 by parasites on the organization of their hosts, both animal and 

 vegetable. Conspicuous instances are furnished by the malacos- 

 tracan Crustacea, in which group further evidence has been 

 brought to light by the work of Geoffrey Smith, f The 

 parasites producing this result belong to several species of 

 the cirripede Rhizocephala, the internally parasitic Isopod 

 Entoniscidae, and the allied externally parasitic Bopyridae. 

 The effect on the general metabolism varies in different groups. 

 In the case of the Brachyura affected by Rhizocephala, ecdysis 

 ceases from the moult at which the parasite becomes external 



* E. B. Poulton, Phil. Trans, vol. 178, B. (1887), p. 311. 

 t Rhizocephala. Fauna and Flora des Gfolfes von Neapel. Monog. (29), 

 190G. 



