On the Anatomy of Lejotodora hyalina. By H. E. Forrest. 827 



at the same time the animal progresses forivard. This forward 

 motion is no doubt due in part to a shghtly backward direction of 

 the down stroke, but I think that it is also largely due to the 

 structure of the antenna. 



As I have pointed out, the flattened oar-like end of this organ 

 is jointed, and therefore flexile. At each downward stroke it bends 

 upwards, at each upward stroke it bends downwards. The setae 

 all taper to the point, so that the base is stronger than the point, 

 and the weakness mcreases gradually upwards: consequently 

 although the normal position is straight, the force of the impact of 

 the water causes it to yield and bend, and this yielding is greater 

 towards the point than at the base, and since they are all united 

 so to speak, into a compact plate by the fringes on each side, and 

 are all acted upon at once, the whole surface of the oar assumes a 

 curved form, and strikes the water obKquely, instead of flatly, and 

 the animal is in consequence driven forward. My meaning will 

 probably be clearer, if I say that the principle which causes the oar 

 of Leptodora to glide forward, is exactly the same as that which 

 causes a rower's oar to glide dowmvards when he dips it into the 

 water obliquely and "catches a crab." At each stroke of the 

 antennae the direction of the slope is reversed, so that both the 

 upward and downward stroke drive it forward. The direction of 

 the hairs which fringe the setae (Fig. 4), is also such that it is 

 easier for the water to pass over than between them. 



The thorax (Fig. 1, h) is that part of the body which bears the 

 legs. It consists of seven somites which have anastomosed so 

 thoroughly as to leave no traces of the original segments, and 

 indeed we can only guess their number from the number of pau-s 

 of appendages. Six pairs of these (Fig. 1, g^-g^) take the form 

 of legs, and are placed in a semicircle around a kind of platform 

 formed by the top of the thorax. Although they vary immensely 

 in length, they are all constructed exactly alike, each consisting of 

 four rounded joints provided with setae. The smallest pair 

 (Fig. 1, g^) is in the middle of the semicircle, and each succeed- 

 ing pair is longer than the last, the sixth being so long that they 

 meet far above the head (Fig. 4). The three smaller pairs are 

 curved upwards and towards the mouth, the three larger ones in 

 the opposite direction towards the last three. 



The flexure of the limbs naturally leads to the supposition that 

 they are used for capturing prey, and their arrangement round the 

 mouth is such that no animal once within their pale could well 

 escape running the gauntlet between those ferocious-looking organs 

 of destruction, the mandibles (Fig. 1, /). These last are the 

 seventh pair of thoracic appendages, modified for manducatory 

 purposes. It is remarkable that none of the limbs are converted 

 into branchiae such as we find in the Daphniadae. The dorsal side 



