tJie Mouth in Sucking Crustacea. 7 



no mentum is evolved from the sphenoid plate, and the tongue, 

 rising from the bottom of tlie cavity of the mouth, appears im- 

 mediately over the edge of the sphenoid plate ; nor do the ordi- 

 nary oral appendages of the third pair (second pair of maxillae) 

 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 maxilla3 and the tongue) their cha- 

 racteristic flattened and foliaceous appearance, and in placing 

 the tongue in a very peculiar jiosition 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 maxillse 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, Idothea3, and Sphas- 

 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 Avood, 

 attack fishing-nets, &c. 



