78 THE CRUSTACEA 



being usually superseded by that of clinging organs. In the 

 Harpacticidae and Asterocheridae the exopodite is retained, though 

 often reduced to a single segment. In the other families it dis- 

 appears altogether. The endopodite never consists of more than 

 two segments, so that in the absence of the exopodite the 

 appendage is usually composed of four segments. In the parasitic 

 families the antennae generally form organs for attachment to 

 the host, provided with a strong curved claw and, in some cases, 

 even with a chelate termination. 



The labrum and metastoma are very variously modified, especi- 

 ally in the parasitic forms, and will be described below in connec- 

 tion with the other mouth-parts. In the free-living forms the 

 metastoma may either form a simple ridge or may be provided 

 Avith a pair of movable lateral lappets. 



The mandibles are especially interesting from the fact that in 

 many Eucopepoda (especially the Gymnoplea) they retain more 

 completely than in any other Crustacea the form of biramous 

 swimming-limbs which they possess in the nauplius. The body of 

 the mandible is formed by the proximal segment, and the remainder 

 of the protopodite, together with the endopodite and exopodite, 

 forms the "palp" (Fig. 8, A, p. 12). Hansen has pointed out the 

 existence in some Gymnoplea of a minute intermediate segment 

 between the body of the mandible and the large segment which 

 bears the endopodite and exopodite. It would seem, therefore, 

 that the body of the mandible in the Eucopepoda, and no doubt 

 also in other Crustacea, represents the precoxal segment of the 

 limb. The endopodite, as in the case of the antennules, consists 

 of, at most, two segments, while the exopodite may consist of 

 five or six. The cutting edge in most free-living forms is armed 

 with teeth and setae. 



The type of mandible just described is universal among the 

 Gymnoplea and is found in some Harpacticidae and Ascidicolidae. 

 More usually, however, among the Podoplea the palp is reduced, 

 as in Cyclops, to a papilla bearing a tuft of setae, or is altogether 

 absent. The form of the cutting edge is variously modified, especi- 

 ally in the parasitic and semiparasitic forms. In many of these the 

 mandibles have a sickle-shaped blade, with the point directed into 

 the cavity of the mouth, but in those forms which have completely 

 suctorial mouth-parts (formerly grouped together as Siphonostomata) 

 the mandibles become simple piercing stylets, and are enclosed 

 within a conical or tubular " siphon " formed by the upper and 

 lower lips. In some cases the siphon may be as long as the body 

 (Acontioplwrus) (Fig. 44, A). The structure of this siphon is not 

 in all cases the same, though it does not seem to be the case, as 

 has been stated, that it is ever formed from the labrum alone. 



The maxillulae (commonly called the maxillae) are most com- 



