CRUSTACEA 387 



Other group, though the Copepoda approach them in this. The ideal 

 malacostracan has twenty somites, including the preantennulary and 

 excluding the telson. Of these, six belong to the head (p. 332), eight 

 constitute the thorax, and six the abdomen. This number is only 

 departed from in the Leptostraca, which have an additional somite 

 at the end of the abdomen. (In the embryos of Mysidacea such an 

 additional somite is present, but in the adult it has fused with that 

 which precedes it.) The female openings are always on the 6th thoracic 

 somite, and the male on the 8th. A carapace encloses the thorax at 



sk.hst. 



Fig. 264. Laura gerardiae. After Lacaze-Duthiers. A, The animal intact, 

 attached to the skeleton of its host, after removal of the soft tissues of the 

 latter. B, A view obtained by opening the mantle along the dorsal side. 

 a. anterior end; Ir. liver, branching in mantle; mtl. mantle; ov. ovary; 

 sk.hst. skeleton of the host. 



the sides. The median eye is vestigial in the adult, and the compound 

 eyes stalked. The antennules are biramous, as they are in no crus- 

 tacean of any other group. The antennae have a scale-like exopodite 

 by extending which the animal keeps its body level in the water. The 

 mandibles have uniramous palps and the part which projects towards 

 the mouth is cleft into "incisor" and "molar" processes. The maxil- 

 lules have two endites (on the first and third joints) and the maxillae 

 four, grouped in twos. The thoracic limbs have a cylindrical, five- 

 jointed endopodite (p. 336), used when the animal has occasion to 

 walk or to grasp large particles of food, a natatory exopodite, and two 



