ARTHROPOUA. 89 



and 4th segments, is the elongated and slit-like mouth (m), with 

 a posterior lip, the metastoma (28 A, mi), from which a pointed 

 lobe grows out on each side, and an overhanging pointed anterior 

 lip, the Idbrum (28 A, lb\ both of which are supported by calca- 

 reous plates. In front of the mouth the sternal region is sharply 

 bent up, and, owing to this cephalic flexure, the anterior append- 

 ages are directed forwards (28, B). The last three pairs of head- 

 appendages are small jaws, covered over by the foot-jaws. The 

 hindmost are the second maxillae (27, E), which resemble the first 

 foot-jaws, but are more delicate. The exopodite and epipodite 

 are fused into a relatively large elongated plate, which lies in 

 the front of the gill-chamber, and has received the special name 

 of scaphognatliite (sg). These appendages overlap the still weaker 

 first maxillae (D), which are devoid of endopodite. Lying at the 

 sides of the mouth are the mandibles (C), in which exopodite and 

 epipodite are absent. The protopodite is a firm, transversely 

 elongated structure, the inner end of which possesses two strongly 

 toothed ridges. The endopodite is a small three-jointed palp. 

 The two segments in front of the mouth bear slender forwardly 

 directed tactile appendages. Those of the 2nd segment are the 

 large feelers or antennce (B). Each of these consists of a proto- 

 podite with two stout cylindrical joints, the proximal of which 

 has a small tubercle on its under surface, upon which is the 

 excretory or renal aperture (28, B). To the protopodite a scale- 

 like exopodite, the "squame," is attached externally, and a very 

 long slender endopodite internally. This is composed of two 

 stout proximal joints and a terminal part made up of a great 

 many small rings, imperfectly jointed together. The small 

 feelers or antennules (A and A') attached to the 1st segment are 

 of much less size. They lie rather internal to the antennae, and 

 are directed forwards. The protopodite is three-jointed, and its 

 proximal joint, which is much the largest, abuts against its fellow 

 in the middle line. It is somewhat trihedral, and on its upper 

 surface is a small longitudinal slit (au), the auditory aperture, over 

 which a horizontal brush of seta? attached to its outer edge 

 projects. The endopodite and exopodite are two short, slender, 

 imperfectly-jointed filaments, of which the latter is rather the 

 longer. 



Between the sterna of the thorax and posterior part of the 

 head a number of cuticular folds project into the body. These 



