170 ON THE STKUCTURE AND 



places by strings attached to buttons. In fact, it is a hydrostatic 

 cushion, inflated by the varying pressure of the blood in the body- 

 cavity beneath it, the strings of the cushion being represented by 

 contractile muscular fibres {7n\ Fig. ii), four or more in number, 

 arising from points of the exo-skeleton on either side the head. 

 If the muscular action exceed the pressure of the blood on one 

 side, the eye will be drawn round on its cushion toward that side, 

 and vice versa; the movements of the eye are the result therefore 

 of a beautiful balance of muscular action and hydrostatic pressure, 

 I have met with similar examples in the insect world. 

 The action of the muscles is probably entirely reflex, in common 

 with most if not all the actions of the creature. I have, on 

 former occasions, in the Note-Books of our Society, represented 

 the muscles as passing round the eye like a rope round a pulley. 

 This, however, I now see to be a mistake. A curtain of connective 

 tissue passes down in front of the eye and confines the distending 

 current of blood within its proper limits. It is only in those 

 specimens whose blood has a red tinge, that the optic sac is 

 rendered evident, its bulging lower surface having been previously 

 mistaken by me for shreds of connective tissue. It may perhaps 

 be thought strange that the optic sac should have become thus 

 separated from the adjacent epidermic tissue of the head, and 

 wholly immersed in the mesoderm of the body, but this is 

 precisely what happens with the whole extent of the nervous 

 cord itself, which by a like process of invagination, eventually 

 becomes separated from the epiblastic tissues which gave it birth. 

 I may add that at one period of its development the eye is 

 seen to consist of a central mass of pigment (Fig. 22), surrounded 

 by large transparent truncated cells, at the bottom of each of 

 which is a crystalline cone in course of formation. Finally, I 

 would ask whether the absence of corneal lenses, accompanied as 

 we see it is by a mobile condition of the remaining visual elements, 

 may not suggest a connexion between these two conditions, and 

 throw some further light on the functions of the cornea in 

 arthropods generally. 



The reproduction of Daphnia has been well described by Sir 

 John Lubbock, in the paper already referred to. All the stages of 

 egg-development may be seen and studied with advantage. In 

 some the rudiments of the eggs only are seen in the ovary, in 

 others the eggs have passed from this into the brood-chamber or 

 receptacle, and in others the egg-covering or vitelline membrane 

 has been cast off, and the embryo young are gradually assuming 

 the mature form. This constitutes the ordinary or agamic process 

 of reproduction, and it will be noticed that I have spoken of these 



