THE MAXILLARY FAN 41 



5. Close up to the lower lip, and as little leg-like as 

 any of the appendages, are placed the first maxillas. They 

 are almost always thin and foliaceous, with few joints, and 

 those few not easy to distinguish. The pattern is ex- 

 tremely variable. The functions are obscure. There are 

 sometimes strong fringing spines which may assist in 

 dividing the food. There are plumose hairs, some of 

 which may be connected with the sense of taste. The 

 position of these organs has also suggested that they may 

 'be useful in preventing the escape of food from the 

 lateral angles of the mouth.' It is with these that some 

 authors close the number of true cephalic appendages. 

 Of those which follow some are frequently, bat none with 

 the same constancy, developed into mouth-organs. 



6. The second maxillae, when present, have generally 

 the same thin flattened character as the first, but their 

 structure is often more complicated. Among other pecu- 

 liarities they have in some of the higher groups a mem- 

 branous expansion or large lamina on the outer side, 

 frequently termed the flabellum or fan, and compared by 

 Milne-Edwards to a ventilating register (see Plate XI.). 

 This species of valve is in constant and rapid vibration, in 

 most cases forcing the water which has aerated the gills to 

 pass out in front, so that a new supply may be introduced 

 from behind ; but in some crabs (as Dorippe, Ranina, Leu- 

 cosia) according to de Haan the water is introduced in 

 front, and passes out behind. Huxley supposes that this 

 valve may represent the epipod, that is to say, the branch 

 which is given off by the first joint of an appendage, or 

 else that it may be a combination of the epipod of the first 

 joint with the exopod of the second. 1 Professor Sars re- 

 gards it as the exopod, 2 while Dr. Hansen considers that 

 it springs from the third joint. 3 In those Crustacea which 

 have the branchias either not enclosed in a chamber or in 

 one remote from the head, this part of the maxilla is either 

 absent or rudimentary (see Plates XVI., XIX.). 



1 The Crayfish, p. 170. (The scaphognathite.) 

 : The Norwegian North Atlantic Expedition Crustacea, p. 21. 

 3 Orcrsigt over de paa Dijmpltna-Togtet indsamlede Krebsdyr, p. 193 

 note, and p. 252. 



