Structure of the Mouth in Sucking Crustacea. 2GS 



short prehensile teeth ; the inner lobe consists of two parts, 

 an anterior harder portion, with a serrate edge, and a posterior 

 bundle of hairy membranaceous lobes, three on the left, two 

 on the right mandible. The teeth of all four lobes are ar- 

 ranged so as to interlock, both the shape of the teeth and the 

 torsion of the lobes towards the under surface and towards 

 one another being accordingly somewhat different on the left 

 and on the right mandible. The structure of the two pairs of 

 maxillae, the first pair with two lobes and palpus, the second 

 pair with two lobes, has often been described. 



19. The second type is met with in most of the Amphipoda 

 known as Lysianassa^ Anonyx^ and O/j/s, placed together by 

 Dana in a separate division under the name of Lysianassina. 



The mandibles are here narrow and high. The pivot-and- 

 socket arrangement behind is the usual one ; but besides this 

 we find, near the apex of the stipes on the upper side in front 

 of the palpus, a club-shaped articular process, with rounded 

 apex fitting into a corresponding acetabulum on either side of a 

 saddle-shaped socket or mortise in the palate, close behind the 

 upper lip, opening towards the mouth. The movement of the 

 mandibles is therefore regulated, not only by the mandibular 

 springs of the lower lip, which are here always present, but 

 besides by the two articulations mentioned, one at either end of 

 the outer margin. This kind of movement is in accordance with 

 the peculiar structure and corresponding use of the outer lobes. 

 These latter do not, as in the Eleutherognatha, form prehensile 

 tongs armed with teeth, but powerful scissors, of which the 

 short, edgewise-set blades cross each other, their cutting parts 

 being wedge-shaped and furnished with an exceedingly sharp 

 edge coated with enamel as hard as glass. With reference 

 to this combination of structure, the Amphipoda of this series 

 may be described as Trochalognatha. 



As a specimen of this type we take the Anonyx lagena^ 

 Kr. Viewed from the side, the head presents especially 

 this difference from the head of Eleutherognatha, that the 

 upper lip forms a projecting hump, the mandibles are much 

 higher and, instead of a pointed triangular outline, present 

 a longer anterior margin slanting downwards ; the mandi- 

 bular springs of the lower lip are also considerably longer. If 

 we examine the head from below, after having removed the 

 antennje and the appendages of the mouth except the mandibles, 

 we observe still better the peculiar shape of the thick protru- 

 ding upper lip, of which the lower margin even forms a separate 

 thicker wall or bolster. This latter covers the upper corners 

 of the mandibular scissors, of which the left blade, furnished 

 with a bifurcate prehensile hook in the lower corner, glides on 



18* 



