Structure of the Mouth m Sucking Crustacea. 301 



joint, which scarcely reaches beyond the first third of the 

 outer lobe, and on its truncate apex carries a considerable 

 bundle of thin tactile setse. The lower lip reaches to the base 

 of the terminal spines of the first pair of maxillge and the 

 inner corners of the outer lobes of the mandibles ; as already 

 indicated, it resembles entirely the tongue in Isopoda, narrow 

 at the base, a little wider at the apex, and there divided into 

 four lobes with small and delicate spines on their apices, the 

 two lateral lobes being longer, obtusely pointed, diverging, 

 and slightly incurved, whilst the two middle lobes are small, 

 pointedly rounded, leaving a narrow split in the middle line 

 of the mouth. The mandibles, too, resemble strikingly the 

 same organs in Isopoda : they are remarkably small, reaching 

 backwards only as far as the middle of the stipites of the first 

 pair of maxillaj, but are nevertheless strongly built in propor- 

 tion. The upper surface of the stipes is in the middle strewn 

 with exceedingly small warts. There is no palpus ; but both 

 lobes are present, though those on the right mandible are not a 

 little different in structure from those on the left. Examining 

 the right mandible from above, the outer lobe is seen to end in 

 five short thick teeth forming a transversal row, of which the 

 innermost is somewhat outward-bent and covered by the 

 inner lobe, whilst the outermost tooth is placed a little above 

 its next neighbour, and this again a little over the nearest of 

 the two middle teeth. Accordingly, when we examine the 

 same mandible from below, the outer lobe is seen to cover the 

 inner lobe to some little extent ; and on account of the arrange- 

 ment of the teeth just described, there appear in this position 

 to be only four on the outer lobe instead of five. The outer - 

 lobe of the left mandible, on the contrary, has six teeth on its 

 apex, placed in two rows, diverging towards the inner side, 

 shorter and much less incurved than those on the two lobes of 

 the mandible of the opposite side, between which they fit in. 

 The differences between the inner lobes of the two mandibles 

 are still more considerable. On the right mandible the ante- 

 rior portion of the inner lobe is of about the same structure as 

 the outer lobe : it terminates in a thick incurved tooth ; and 

 its anterior margin carries three thick teeth, which are placed 

 on a lower level than the terminal tooth, but otherwise resemble 

 it in shape and size ; behind this anterior portion, with its 

 hard enamel and armature of teeth, follows a thin closely 

 ciliated membranaceous portion, of which the apex is cleft into 

 six thin digitiform lobes. On the left mandible, on the con- 

 trary, the anterior part of the inner lobe lacks both enamel 

 and teeth, and is represented by a short protuberance, with 

 small warts on its sides and with a small slightly undulated 



