LEPTODORA HYALINA. 259 



tribes or " telescopes," t t, furnished each with a lens at the outer end, 

 and a layer of black pigment to receive the image at the inner end ; this 

 pigment extending partly up the sides of each tube to separate them 

 from one another, and giving the appearance of a black centre to the 

 whole eye. Now, each of these " telescopes," if it were stationary, would 

 be only suited for seeing distinctly an object in the one direction of the 

 axis of the telescope, as shown by the dotted centre lines, u u, in Fig. 8 ; 

 and other objects that were lying between these directions would be only 

 imperfectly seen, or not at all visible. But, if each telescope, instead of 

 being stationary, is given a range of lateral motion sufficient to take in that 

 space, as shown by the shorter dotted lines, v v, there will be a direct 

 vision of every object in succession, and this is exactly the motion that is 

 possessed by the eyes of Leptodora and Daphnia, which constantly roll 

 about through a small range of motion, by an apparently involuntary 

 action. It is now suggested that this is an explanation of that singular 

 motion of the eye, which is only needed in such cases as Leptodora and 

 Daphnia, in which the head is rigidly fixed by the stiff armour-casing 

 that does not admit of any lateral movement of the head for shifting 

 the direction of sight of the fixed eye. In the numerous cases of insects 

 with fixed hemispherical eyes studded with separate lenses, the neck is 

 sufficiently flexible to allow of the head with its fixed eyes being moved 

 about as required for obtaining complete vision in different directions. 



The thorax of Leptodora is furnished with six pairs of limbs or 

 " foot-jaws," ranged on each side of the mouth, for bringing food to the 

 mouth ; these diminish successively in size from the back pair, which 

 are long and four-jointed, to the front pair, which are reduced to simple 

 palpi. The mouth (drawn enlarged in the front view, Fig. 4, and the side 

 view, Fig. 5) is surrounded by the foot-jaws, and covered by a large hood 

 that moves up and down, forming the labrum or upper lip ; the 

 alimentary canal leads direct from the back of the mouth, and passes 

 down in a straight uninterrupted line through two-thirds of the length 

 of the abdomen, and then opens into an enlarged intestine or stomach. 

 There has not been any trace of food observed to remain in this alimen- 

 tary canal, which appears to act throughout simply as an sesophagus. 

 At each side of the mouth is a curved pointed mandible, (drawn separately 

 in Fig. 6,) each armed with three short barbs near the point, and the 

 base of the mandible is bedded upon a mass of muscle. The pair of 

 mandibles have a transverse movement in the mouth, their barbed ends 

 crossing one another and then separating alternately, with an action 

 similar to the mandibles of caterpillars and other larvaB ; the barbs are 

 on the upper side only of the mandibles, and do not interfere with their 

 rubbing together in this movement. This pair of mandibles form in 

 position a continuation of the line of limbs or foot-jaws, and they are to 

 be looked upon morphologically as a greatly modified pair of limbs ; in 

 the same way as in the large crustacean, the lobster, one pair of limbs 

 are modified in the opposite direction into the large crushing-claws. 



The heart (shown in Figs. 4 and 5) is a large contractile vessel, 

 situated at the back of the thorax, and consisting of a singular ribbed 



