322 Dr Coldstream 07i the Structure and 



the formation of the mouth ; the other is narrow near its base, 

 and increases in breadth anteriorly ; its inner edge is straight 

 and inflected, so that the surface which bounds the oral fis- 

 sure is broad. It, as well as the other parts of the jaw, is co- 

 vered with short stiff hairs. The internal jaws (Fig. 10) are of 

 more simple structure. They are of a lengthened quadrangu- 

 lar fdrm. Their free extremities are tipped with bristles. The 

 mandibles are placed obliquely, and in such a manner as that 

 their sharp points project from the most advanced part of the 

 head when the jaws are moved aside. Their form is shewn in 

 Fig. 11, and their attachment to the corselet in Fig. 12, where 

 a a point out the situation of the mandibles. Each is provided 

 with two very sharp and hard points, coloured brown, one of 

 which is in contact with its fellow of the opposite side, the other 

 projects anteriorly. The mandibles are about ^^^th of an inch in 

 length ; their points are perfectly smooth ; they are furnished 

 with palpi, one of three articulations being attached to each. 

 These are represented in Fig. 11.* The mandibles are doubt- 

 less the boring organs ; and it is easy to conceive how powerful 

 they must be, notwithstanding their extreme minuteness. They 

 have, however, fewer peculiarities of form and structure than 

 one might expect to find in an animal whose habits are so dissi- 

 milar to those of other Crustacea. Here, as in many other cases, 

 we see that, by a very small alteration in structure, the organ 

 .is adapted to a purpose differing much from its usual appropria- 

 tion in other animals. It is not by creating new organs that a 

 new function is fulfilled, but very df>en merely by changing, 

 and that but slightly, the forms of organs already in existence. 

 In some of my examinations of the oral organs, I have indis- 

 tinctly seen what seemed to be funnel-shaped lips within the 

 mandibles, fitted for protrusion beyond the jaws, and apparently 

 for suction. Of this, however, I am uncertain ; nor can I de- 

 scribe particularly the course or the form of the oesophagus ; 

 but, connected apparently with that organ within the head, and 

 hanging down in the cavity of the body as low as the fourth or 



• M. Desmarest, in his valuable systematic treatise on crustaceous animals 

 (1825), places the limnoria in the order Isopoda of the class Edriophthalmia, 

 and gives, as one of the characters of that order, that the species included in 

 it have no mandibular palpi. I am satisfied that they do exist in the limnoria. 



