272 THE ANATOMY OF INVERTEBRATED ANIMALS. 



to be undeveloped, and the joints of the endopodite are com- 

 pletely foliaceous. The somite which supports the mandibles 

 is, to a great extent, membranous in its sternal region ; it is 

 united with the corresponding region of the first maxillary 

 somite, itself represented merely by a narrow, distinctly cal- 

 cified, band, in front of the second maxillary sternum, by mem- 

 brane only. In this membranous space the elongated aper- 

 ture of the mouth is situated. 



On each side of and behind the mouth are two little 

 elongated oval calcified plates, between which an oval pro- 

 cess, setose at its extremity, proceeds downward and for- 

 ward, and lies in close apposition with the posterior face of 

 the mandible of its side. This is one-half of \y hat is termed 

 by most authors the labium, but, to avoid confusion with the 

 labium of JTnsecta, from which it is wholly different, it may 

 be called the metastoma (Fig. 72,/). It obviously answers 

 to the structure so named in the Copepoda. 



The mandibles fill up a large space in the sternal mem- 

 brane, with which their edges are continuous on each side of 

 the oral aperture ; externally, the sternal membrane bends 

 suddenly downward into the pleural ridge, continuous with 

 the branchiostegite of the carapace, and becomes calcified ; 

 while, anteriorly, it is very difficult to say where the mandi- 

 bular sternum terminates. In front of the mouth the sternal 

 membrane becomes developed into a large median lobe, con- 

 taining three small calcified plates on each side of the middle 

 line. This is the labrum (Fig. 72, e). 



The mandible itself (Fig. 71, A) is thick and strong at its 

 inner end, where it is divided by a deep excavation into an 

 upper and a lower portion (a, b), the edge of each being 

 toothed. The outer division of the mandible extends along 

 the whole width of the somite, and tapers to its extremity, 

 which presents an articular head, the outer condyle. At- 

 tached to its anterior margin is the palp (o), which represents 

 the terminal joints of the mandibular endopodite. The ex- 

 opodite and the epipodite have no representatives in this ap- 

 pendage. Superiorly, the outer portion of the mandible is 

 concave, and its posterior edge gives attachment to the cal- 

 cified tendon of the adductor mandibulre (d). 



In front of the labrum and mandibles is a wide, somewhat 

 pentagonal area, prolonged into a point in the middle line 

 forward, and presenting a small spine on each side ; this is 

 the epistoma (Fig. 72, B, I), and it is chiefly, if not entirely, 

 formed by the sternum of the antennary somite. On each 



