748 SUMMARY OF CURRENT RESEARCHES RELATING TO 



cells in the lateral eyes aie the exceedingly elongated ectodermic cells, 

 each with its own nucleus, which closely inrlent the outer parts of the 

 rod-bearing cells. The entire structure of such elongated cells can only 

 be brought out by maceration of the retina, for sections are necessarily 

 misleading. The similarly named cells in the median eyes are also five 

 ectodermic cells not derived by secondary migration from the mesoderm. 

 The number of retinula-cells in the ommatidia of the lateral eyes is not 

 constant, but very variable. The axial cavity inclosed by the rods, and 

 by the rhabdom, is not empty, but is occupied by the axial process of the 

 central ganglion cell. 



f. Crustacea. 8 



Senses and Habits of Crustacea.* — Mr. W. Bateson, in the course of 

 his investigations on the perceptions of fishe=!, has made some interesting 

 by-observations on Crustacea. All in the tanks at the Plymouth 

 laboratory, except Carcinus msenas and Portunus depurator, are more 

 active by night than day, and many rarely come out by day at all. 

 Excepting the shrimps, nearly all the individuals of the other forms 

 observed have each its place to which it retires when morning comes, 

 and in which it remains during the whole day. To the shrimp it is of 

 paramount importance to know the difference between night and day, for 

 it is not safe for it to hunt till darkness comes. Strangely enough it 

 seems that this knowledge is not obtained by the eyes, or at all events 

 not entirely through them, for there is no observable difference when 

 the eyes are extirpated. Prawns, shrimps, and others find their food 

 almost exclusively by scent, but they do not seem to have a very accurate 

 knowledge of the direotion of it ; it is not even certain that they can see 

 each other. In some cases the eyes were observed to be particularly 

 sensitive to shadows. Though it seems probable that the sense of smell 

 is by the antennules in shrimps, at any rate, it is not exclusively so, 

 for a shrimp with no antennules will hunt if a piece of worm be put 

 very near it. 



A very interesting description is given of the method by which 

 certain crabs fasten pieces of weed and so on to their backs and appendages. 

 The crab takes a piece of weed in his two chelae, and neither snatching 

 nor biting it, deliberately tears it across as a man tears paper with his 

 bands. He then puts one end of it in his mouth, and, after chewing it 

 up presumably to soften it, takes it out and rubs it finely on his head 

 and legs until it is caught by the peculiar curved hairs which cover 

 them. The whole proceeding is most human and purposeful. The 

 various substances used are nearly always symmetrically placed on 

 corresponding parts of the body. Curiously enough not only are all 

 these complicated processes gone through by night as well as by day, 

 but a Stenorhynchus cleaned and deprived of sight will immediately 

 begin to clothe itself again with the same care and precision as before. 



Function of Spines of Crustacean Zocese.j — Mr. W. F. E. Weldon, 

 by comparing the behaviour of such zooese as have long spines and of 

 those that are devoid of them, has been led to suggest a function for 

 these organs. A larva that has them swims in an absolutely straight 

 line towards the light, moving with great rapidity, and neither changing 

 direction nor losing equilibrium during a journey of several feet. Larvae 



* Journ. Marine Biol. Assoc, i. (1889) pp. 211-4. f T. c, pp. 169-70 (1 pi.). 



