the Mouth in Sucking Crustacea. 5 



The appendages of the mouth are consequently prolonga- 

 tions of the pleurge of the head. In many Crustacea more or 

 less marked pleural folds may be observed, one fold for each ap- 

 pendage, indicating that the head is composed of as many rings 

 as it possesses appendages of the mouth. 



The appendages of the mouth (oral limbs) consist typically 

 of the following parts : — 



(1) An articular fold near the base, the hinge (cardo). The 

 corresponding articular fold at the base of the legs (or the 

 limbs organized for locomotion) has been called " trochantin." 

 In those insects where the coxaj at their base are surrounded 

 by sockets, the trochantins form the condyles of the coxae, being, 

 as well as the latter, free of the epimeron. 



(2) A stem (stipes), corresponding to the coxse of the locomo- 

 tive limbs, which is developed in proportion to the require- 

 ments of the lobes, its destination being to carry the latter and 

 to accommodate the muscles by which they are moved. 



(3) Three free lobes, at the end of the stem, of which the 

 two innermost (mate) serve for subdividing and handling the 

 food and are therefore modified in accordance with the na- 

 ture of the food. The third and outer lobe is the continuation 

 of the limb as such, and corresponds to those divisions of the 

 locomotive limbs which follow the coxa. When it is elongated 

 and jointed, or shaped as a leg, it is denominated palpus ; and 

 its destination is then either to carry organs of sense, to pro- 

 duce currents in the water about the mouth, to cleanse the 

 organs of the mouth, to serve as instrument of prehension, or 

 some other such function. 



4. A fundamental difference between Insects and Crustacea 

 is now to be observed, in the relations of the first pair of oral 

 appendages to the side pieces of the skull. 



In Crustacea the mandibles do not exceed that point of de- 

 velopment which is attained by the other appendages of the 

 mouth, and consist like these of hinge, stem, and lobes. 

 Their flexors are also, as far as the head is not united with 

 the body, attached to the hypostoma, and their movement is 

 an oscillation, which has for its axis the whole exterior margin 

 of the stem, and which sometimes is regulated by an imper- 

 fect articulation at the exterior and posterior corner of the 

 stem. 



In Insects, on the contrary, both hinge and stem coalesce 

 with the pleur£e of the head, and their proper muscles are not 

 at all developed. The middle lobe alone remains, and articu- 

 lates with the side pieces of the skull by an uj)per and a lower 

 condyle, whilst its muscles fill a great portion of the side 

 piece. The inner lobe is only very rarely developed, and is 



