REPORT ON THE CRUSTACEA MACRURA. 



xxxi 



Sc 



sharp point, and the inner side flattened out to a thin foliaceous plate, broad at the base 

 ami gradually narrowing to its extremity, or to near the apex of the outer margin which 

 is generally separated from it as a free process. 



It is, however, amongst the Dendrobranchiata that this appendage is seen in its fullest 

 development. In Pen&us, Aristeus, &c, it is large and broad, with a 

 small tooth on the outer margin. In Benthesicymus and other deep- 

 sea forms it is broad and of extreme tenuity, having only the feeblest 

 representation of the external marginal tooth. In Sicyonia, on the 

 contrary, the outer margin is intensified to a strong and powerful sharp- 

 pointed spine, and the inner foliaceous plate is reduced considerably 

 in size and thickness. 



In the Phyllobranchiata the scaphocerite is longer than in the 

 Trichobranchiata, but not so broad generally as in the Dendro- 

 branchiata. In some genera, as Oplophorus, it is produced to a sharp 

 point by the strengthening of the outer margin and the reduction of 

 the foliaceous plate of the inner side. 



If we judge of the utility of the scaphocerite by its structure, 

 there can be little doubt that when developed as a large foliaceous 

 plate it is of much value in helping to maintain the animal upright 

 when swimming, preventing it from falling into an inverted position 

 as seen in the Amphipoda and other Crustacea, where it does not 

 exist or is only feebly represented, as in some of the Astacidea. 



When it is produced to a sharp point, as in Sicyonia (PI. XLIII. 

 fig. 3c"), Oplophorus (PL CXXVII. fig. c), Acanthephyra (PI. CXXV. fig. lc), Thalasso- 

 caris (PI. CXVII. fig. lc), &c, it is evidently used as a weapon of offence. In these genera 

 the teeth affixed to the outer extremity of the second joint of the peduncle, which are 

 o-enerally of little importance, are developed to a greater extent, and fulfil an important 

 office by guiding the scaphocerite into a corresponding groove, where they support it. 

 In some instances, as in Sicyonia, they lock it into a fixed position, and thus increase 

 its power as a weapon of offence. In the Astacidas, where it is sharp-pointed and 

 strong, it is too short to be useful as an offensive weapon, and probably is of value only 

 in protecting the sensory organs. 



In Hemipenseus the scaphocerite is very large and broad, and the outer distal tooth 

 is small ; and in some specimens the distal margin is considerably thickened, and the 

 hairs are wanting ; this condition appears rather to be the result of some exceptional 

 state than a normal condition (vide p. 304). I am inclined to believe that this organ 

 may be used for such a purpose as that of disturbing the muddy bottom over which it 

 lives, with the object of procuring food, and that the constant gentle friction so produced 

 would first remove the marginal hairs, and then induce such irritation as to cause this 



Pto 



Fig. IX. — Second an- 

 tenna. Phe, phynia- 

 eerite ; 1,2,3, 4, 5, joints 

 of peduncle ; .Sc, scapbo 

 cerite ; 6, flagellum. 



