the Mouth in Sucking Crustacea. 7 
no mentum is evolved from the sphenoid plate, and the tongue, 
rising from the bottom of the cavity of the mouth, appears im- 
mediately over the edge of the sphenoid plate; nor do the ordi- 
nary ibel dodilictlaiies of the third pair (second pair of maxillz) 
enter into any combination with the tongue as they do in Insects, 
but remain separate, each on its own side of the sphenoid plate. 
The consequence is that the tongue occupies a far more advanced 
place among the organs of the mouth than in Insects. Whilst 
thus the first great peculiarity of the head in hedriophthalmous 
Crustacea (its being finished off underneath by labium-like 
maxillipeds) exercises a very marked influence on the struc- 
ture of the mouth, causing, so to say, a pressure from beneath, 
the second great feature distinguishing their head from that of 
Insects, viz. the free position of the mandibles with regard to 
the skull, produces a similar pressure from above, and both 
together result in imparting to the intervening organs of the 
mouth (the two pairs of maxille and the tongue) their cha- 
racteristic flattened and foliaceous appearance, and in placing 
the tongue in a very peculiar position to the mandibular lobes, 
determining its form once for all. ‘The complete severance of 
the Insect head from the body, which entails its being finished 
off from beneath by the combination of the third pair of oral 
appendages, the mentum and the tongue, into a labium, finally 
its considerably increased thickness, which is caused by the 
coalescence of the stems of the mandibles with the side pieces 
of the head—all these circumstances cause the maxille and the 
tongue to be placed on a so much lower level than the mandi- 
bles (supposing these to lie horizontally), that the tongue re- 
tains free space to develope itself in accordance with manifold 
and various secondary considerations. But in the head of Crus- 
tacea the first pair of oral limbs, being entirely separate from 
_ and outside the side pieces of the head, are depressed into a 
lower level than that of the sphenoid plate, which lies very high; 
and consequently the tongue is placed above the mandibular 
lobes, and cannot possibly have any other than a deeply bifid 
shape, as it would otherwise close the aperture of the mouth. 
6. The mouth of biting Isopoda presents three principal 
types, which agree in this, that the oral limbs are placed in a 
row slanting outwards and forwards on either side of the 
sphenoid plate, each independent of its neighbours. 
The first type comprises Onisci, Aselli, Idothes, and Sphee- 
romata. It is essentially the same as the one we meet with 
in the majority of Amphipoda, that is, upon the whole in those 
Edriophthalmia which live near the shore or on the bottom of 
the sea, and feed upon carrion or vegetable food, gnaw wood, 
attack fishing-nets, &c. 
