REPORT ON THE CRUSTACEA MACRURA. \xi 



animal, since they are invariably present, and undergo modifications with sexual and 

 specific variations ; the inner, on the other hand, is of less importance, and seldom 

 varies except in relation to length, in some forms being reduced to a minimum, or, as 

 in Lucifer, it is wanting altogether. 



Although in all Decapod Crustacea the first pair of antenna? consists of a peduncle 

 and one or more flagella, yet the organ undergoes modifications in the different orders. 



In the Trichobranchiata it may be considered as typical of the Macruran form, such 

 variations as exist being common to the other divisions. 



The most simple form exists in the Synaxidea, of which that in the Palinuridse may 

 be taken as the most normal. In these the peduncle consists of long narrow cylindrical 

 joints, projected on an exposed portion of the antennal somite, and terminating in two 

 slender flagella of nearl)" equal length. The first or basal joint is generally longer than 

 the others, and increases in diameter towards the articulation with the somite ; within 

 this enlarged portion an acoustic organ exists, that undergoes modifications in the 

 different genera. In Palinurus, Homarus, and Astacus the perforation is long, narrow, 

 and slit-like, the aperture being scarcely appreciable, and opens into a calcified chamber, 

 more or less filled with particles of sand, which are voluntarily placed in position by the 

 animal soon after casting its exuvium, 1 and although the joints of the peduncle are 

 cylindrical or nearly so in Palinurus, Ibaccus, Homarus, &c, yet in some genera of the 

 Astacidea they undergo certain modifications, as, for instance, in those animals in which 

 they are laterally compressed, the approximating sides being flattened against each other, 

 and this is carried to such an extent in some genera of the Eryonidse (as in Willemcesia), 

 that the inner margins are pressed together, forced upwards, and thus form a vertical 

 ridge in the median line. 



The second and third joints of the peduncle are of little importance, and apparently 

 only serve as carriers of the terminal flagella. In the ordinary or most simple form they 

 are merely cylindrical joints, but in some species they are broad and short, having the 

 distal angles produced to strong teeth. 



Each of the two flagella arises from its own distinct base at the extremity of the third 

 joint, one oblicpiely above the other, that on the outer and upper side being the more 

 robust, and built up of a number of short rings or articuli, which are more or less 

 abundantly furnished with protective hairs or spines, and amongst them are always 

 a considerable number of flexible membranous tube-like cilia, that vary somewhat in 

 form corresponding with other generic characters. 



These membranous cilia were, I believe, first pointed out by n^self in a memoir 

 On the Homologies of the Carapace and on the Structure and Function of the Antennse 

 in Crustacea, 2 in which it is stated that the cilia " are always larger than ordinary hairs, 



1 This was first pointed out by Dr. Farre in 1843, Phil. Trans., vol. cxxxiii. pp. 233-242. 

 i Ann. and Mag. Nat. Hist., July 1855. 



