448 MR. P, H. GOSSE ON THE STRUCTURE, FUNCTIONS, AND HOMOLOGIES 
cheeks, into which they are articulated. As in Insects, their usual form is curved, 
with the convexity outwards, and the extremity variously notched or dentated. 
Their motion, as in that great Class, is chiefly a shears-like opening and shutting, in 
the horizontal plane ; combined with a kind of rotatory action, which, according to 
Mr. Ktrby, the mandibles of some insects possess*. The same excellent entomo- 
logist enumerates several instances, in which the mandibles of Insects are not symme- 
trical, the one being developed more than the other ; or, at least, differing from it in 
form -f-: — a circumstance in which these organs curiously agree with the unsymme- 
trical mallei in Mastigocerca, &c. 89 to 91). 
142. A glance at the trophi of Diglena, Alhertia,OY Synchaeta, is sufficient to show 
that the incus with its rami is the homologue of the rnaxillse, since they are placed, 
in these genera, nearly in the same plane as the mallei (= mandibulee), and within 
them. There are not wanting, however, numerous instances, among Insects:|:, in 
which the direction of the maxillae is not parallel to that of the mandibles, but 
inclined to it ; as is more generally the case in the Rotifera*^. 
143. The maxillae are much more constantly present, in Insects, than the mandibles ; 
the latter being either greatly deteriorated or entirely wanting in important groups, 
as in Lepidoptera, Aphaniptera, and some Diptera ; and this agrees with the evanes- 
cence of the mallei in many Rotifera, while the incus is almost invariably present. 
144. The rami of the incus, then, form the maxillae proper; but whether the 
fulcrum answers to the two cardines (Kirby), soldered together, or to the labium, I 
am not prepared to decide. I incline, however, to the former hypothesis ; since, in 
Scaridium, as we have seen (§ 93), there appears to be a lower lip, though the 
fulcrum is large and distinct. 
145. In Diglena, and its allies, the manducatory apparatus approaches most 
nearly to that of a predaceous insect ; the rami, in particular, being almost the 
counterparts of the maxillae of a carnivorous beetle. The series of bristles, with 
which the inner surface is fringed (§ 80), aids the resemblance. Asplanchna also 
presents an instructive example of resemblance in these organs. Here the extremity 
is two-lobed (fig. 59), as in many beetles ; while the interior edge is, in A. priodonta, 
cut into numerous teeth. 
146. The lips {lahrum and labium) seem to be lost in all the species which have 
the mastax permanently inclosed ; but in such as can protrude the jaws, the upper 
and lower margins of the mastax form distinct edges, more or less moveable by 
retractation and protrusion. In some instances I have been able to see these margins 
(which I must regard as representing the lips), or at least one of them; as in idyn- 
chceta ('^ 76)5 Polyarthra ('^ 79), and Notommata aurita (§ 52) ; but, in Scaridium, 
* Kirby and Spence, Int. to Ent. iii. 431. f Ibid, iii, 432. J Ibid. iii. 439. 
§ “ The mandibles of the larvse of Tipulae, which are transverse and unguiform, do not act against each other, 
but against two other fixed, internally -concave, and externally-convex, and dentated pieces” (Kirby and 
Spence, iii. 122). 
