162 ON THE STRUCTURE AND 



not easy to recognize in Daphnia. In no portion of the adult is 

 the segmentation of the body so clearly visible as it is in a lobster 

 or crayfish ; and the great extension of the carapace — forming as 

 it does a bivalve shell, enclosing the whole of the thoracic and 

 abdominal regions, — obscures it if possible, still more, so that the 

 number of somites, or divisions of the body, can only be ascer- 

 tained inferentially, and chiefly by the appendages they bear. 

 These are enumerated by Baird, in his history of the British Ento- 

 mostraca, as follows : — The inferior or great antennae, the superior 

 antennae, the mandibles, the maxillae, and five pairs of feet.*" I 

 do not propose to enter into a full description of all these organs, 

 but together with the labrum and the abdominal termination of the 

 body, they will claim our first attention. 



The great antennae are the best known : they are the in- 

 ferior antennae of Baird. They are the sole organs of locomotion, 

 thus offering a striking contrast to the sensory functions fulfilled 

 by the corresponding limbs of the higher Crustacea. They are 

 moved by three powerful muscles {m in m, Fig. 4), inserted in 

 the integument of the head. In the two branches in which 

 they terminate we may recognize the exopodite and endopodite 

 of the limbs of the lobster or crayfish. The joints are furnished, 

 as we all know, with beautiful plumose setae. It is somewhat 

 curious that these setae are always wanting at the junction of the 

 second and third joints of the posterior branch in the antennae 

 of Daphnia rotunda, a species common about London. 



Just below the beak with which the head terminates, (which, by- 

 the-way, must by no means be mistaken for a mouth,) we find the 

 superior antennae. These are inconspicuous organs in the female 

 (see Fig. 11), but are much larger in the male and in the embryo 

 young. They are usually terminated by a number of short, stiff 

 setae; and a large nervous ganglion in connexion with them at the 

 base of the head, shows them to be sensory organs. Below the 

 superior antennae, and just covering the mouth, we find the fleshy 

 upper lip or labrum (/r. Figs. 3, 4, and 11). We may have to look 

 for this rather closely at first, as it lies within the anterior margin 



* A fuller description of their form will be found in Baird. 



