332 ASTACOCROTOX, A XEW TYPE OF ACAEID, 



seem to be the duct is a g-ranular mass contained in the cavity of the proximal 

 joint between the muscles. It consists of a number of lobes which converge 

 towards the distal end to form a vesicle which narrows to form the duct. The 

 proximal joint may be described as oval, somewhat compressed laterally, and 

 narrowing distally. Its long axis is not parallel with that of the capitulum, the 

 proximal end being dorsal to the pharynx and the distal near the mouth. At its 

 oral end it lies in the roof of the mouth cavity, and behind that it develops a 

 knob which articulates with the ehitinous element which I joropose to term the 

 fulcrum. The fulcra are of general, if not universal, occurrence in the Aearidae; 

 but they do not appear to have had any name given to them that could be of 

 general application. In tracheate forms such as Trombidium and the traeheate 

 Hydraehnids they are hollow, the cavity which each encloses being in continuity 

 with the tracheal system. In this connection they are termed the "drittes kiefer- 

 paar" by Haller (1881) "sigmoid ehitinous piece" by Michael (1895) "zweite 

 Luftkammer" by Henking (1882). Each not only supports the corresponding 

 chelicera, but gives origin to the muscles by the contraction of which the move- 

 ments of the latter, as a whole, are effected, 'llie fulcra articulate with the 

 ehitinous "bridge." The middle part of the latter lies on the aboral side of the 

 salivary receptacle in which the right and left main salivary ducts unite to open 

 into the mouth. It is produced downwards and outwards in two strong ehitinous 

 bars which fuse with the cuticle of the lateral wall of the capitulum at its base. 

 The fulcra in Astacocroton are strong, solid, curved rods of dense chitin, which, 

 originating close together where they articulate with the chelicerae and the bridge, 

 diverge as they pass backward and downwards like the limbs of an inverted V. 

 From each arises a set of muscles passing backwards and upwai'ds to be inserted 

 into the proximal end of the basal podomere of the corresponding ehelicera. When 

 these muscles contract they must cause the rotation or tilting forwards in the 

 vertical plane of the chelicerae on the fulcra, and this must result in the move- 

 ment by which the harpoon-like free ends are made to pierce the gill of the 

 crayfish. 



In general it may be said that the structure of the capitulum and the cheli- 

 cerae agrees well with that of these parts in Trombidimn and the Hydrachnida. 

 'Ilae most noteworthy difference seems to lie in the fulcra. In Trombidium and 

 the tracheate Hydraehnids, instead of being solid rods, these, as already stated, 

 are hollow and dilated into the form of air-chambers with ehitinous walls con- 

 nected with the tracheal system, while still retaining the same essential relation- 

 sliip with the bridge, the chelicerae and their muscles. 



The pedipalpi (PI. sxxvi., fig. 1, pd; figs. 4 and 5) are a little longer than 

 tlie capitulum, stout, composed of five joints or podomeres, of which the second 

 is the largest. The last, which is much narrower than the rest, is provided ter- 

 minally with six spines, four of which are hooked; the penultimate bears a spine 

 on the inner side of the distal end. The entire appendage is habitually curved 

 downwards in adaptation to its function of grasping the gill-filaments and sus- 

 pending the parasite. 



The position of the leg-s (PI. xxxvi., fig. 1) in the full-gTown animal is a 

 very marked feature, the last pair being far in front of the middle of the body — 

 in fact, in large specimens not more than about a fifth of the entire length from 

 the anterior end. This is associated with the great distension of the body with 

 the ingested crayfish blood and the accumulated mass of eggs, but begins at a very 

 early stage. 'I'he epimera (coxae) are subequal, the fourth a little longer than 

 the rest, in contact with one another by their edges, but not fused, except the 



