MONOGRAPH OF THE EXISTING CRINOIDS. 627 



In Hippolyte the young larva is at first a colorless creature living at the surface 

 of the sea, and when this is carried inshore it attaches itself to the first seaweed it 

 meets. The skin is in such a sensitive condition that within a week a complete 

 resemblance in color is brought about, whether the seaweed background be red, 

 green, or brown. 



Potts believes that the resemblance in coloration between Synalpheus and its 

 host is a similar phenomenon ; but it is less perfect because natural selection has 

 not been brought into play to the same extent, if at all, as in Hippolyfe, to which 

 the seaweeds offer a holdfast, but not a complete refuge such as the crinoid affords. 



In Synalpheus generally both members of a pair are similarly colored. This 

 is a phenomenon which Potts believes may be explained in two different ways : 

 Eitlier mating takes place early, before the pigment pattern has been finally deter- 

 mined and the same factors act equally on both, producing a similar pattern, or 

 else there is assortive mating. Possibly the truth lies in a combination of botli 

 explanations; if so, the exceptions where mates are dissimilarly colored are due 

 to the breaking down of the rule of a.ssortive mating, or to the existence of indi- 

 viduals which are not able to assimilate themselves to their background. 



With regard to the habits of S. Itrucel, Potts states that it is usual to find the 

 male and female lying side by side on the surface of the disk, but when disturbed 

 they take refuge between the pinnules or on the dorsal surface of the arms. They 

 move about quite freely, but they can guard against forcible detachment by digging 

 the claws of the thoracic legs into the soft flesh of the disk or bj^ clasping the pinnules 

 or arms of the crinoid. The chelae are less effective for maintaining a hold than the 

 thoracic legs, though it is to be noticed that these can not be said to be specially 

 modified for this purpose. Thej' are provided with two sharp claws, but this pro- 

 vision is also made in cases where the alpheid has no such commensal habits. 



When removed from the crinoids they swim about very rapidly, but return 

 as soon as possible to the shelter of the host and cling to it as before. 



They exhibit reactions to light and to touch in a very marked manner. 

 Alpheids placed in a glass vessel always cluster together on the side of the vessel 

 away from the light. Besides being negatively heliotropic, they are strongly 

 thigmotropic, for when the finger is introduced into the water it is instantly em- 

 braced by the thoracic legs of the alpheid. In the absence of any foreign object the 

 alpheids embrace one another, so that a number left together in a vessel soon look 

 like a mass of swarming bees. 



Potts notes that there seems to be a limited faculty for color change. One 

 individual with wide stripes of pigment became lighter toward night, darker again 

 at day. Unfortunately he did not make any extended observations on this point. 



STNALPHErrS, SP. 



Mr. H. C. Chadwick states that a specimen of Comanthus parvicirra from the 

 Gulf of Manaar. Cejdon, which when living was a deep olive brown with the tips 

 of the pinnules yellow, had living upon it an alplieid which was olive brown striped 

 with gray. 



