ON OUR PRESENT /KNOWLEDGE OF THE CRUSTACEA. 65 



in aquatic Cnistacca, and whicli, from their supposed connexion with the 

 sense of hearing, I have elsewhere denominated aural cilia. The secondary 

 branch is less important, and frequently divides into two or more rami. 

 Sometimes these flagelliform branches are reduced in size to a minimum 

 amount, and this generally corresponds with the highest character of tlie 

 organ ; for it appears to be in inverse ratio — the longer and more extensive 

 the character of the terminal ilagella, the less developed is the structural 

 condition of the organ of sense contained within the peduncle ; and, on the 

 other hand, the more developed the sensational organ, the feebler and less 

 numerous is the organism and less antenna-lilve is the general character 

 of the distal portions of the appendage. To this very constant condition in 

 the aquatic forms of Crustacea we have a variation in the terrestrial 

 species. In the genus Onisms and allied forms of Isopoda, as well as in 

 the littoral varieties of Amphipoda, such as Talitnis, Orchestia, &c., the first 

 pair of antennae are reduced to a minimum proportion consistent with their 

 presence, without any increased importance in the structural condition of the 

 peduncular joints, as far as I have been able to ascertain. 



In the highest types of Crustacea the coxal joint is considerably enlarged 

 {vide pi. i. fig. 8 h, lleport for 1875), and contains within it a complicated 

 chamber and highly developed organ of sense ; while in the Macrurous forms 

 a less complicated chamber exists, with an external opening into which 

 small grains of sand find their way : in others, as first shown by Professor 

 Huxley in a species of Stomapod, well-developed forms resembling otolithes 

 are present ; this Dana has observed, and I have been enabled to confirm in 

 a species of Anchistia from Australia (PI. II. figs. 13 & 14). 



In some genera, as My sis among the Stomapoda, they vary in form 

 according to sexual distinction. The male animal has the two terminal 

 flagella feeble and slender, while a fasciculus of strong hook-formed hairs 

 are planted on the inner and lower angle of the most distal extremity of the 

 second joint of the peduncle, while a similar but less powerful group of spine- 

 like hairs are planted on a strongly projecting process on the inferior distal 

 extremity of the first joint (PI. II. fig. 15). There are other hairs implanted 

 on the lower margin of this joint of a very delicate ciliated character. The 

 peduncle of this antenna is very powerful, and there can be little doubt but 

 that it is useful as an organ of prehension, most probably employed in se- 

 curing the mate. These several facts are demonstrative evidence that the 

 first pair of antenna are connected with the acoustic properties. 



Of this I purpose treating, as well as discussing the observations made by 

 Dr. Hcsen in his researches (published in 1864) on the auditory organs of 

 the Decapod Crustacea, when I report on the internal structure of the animal. 



Contrary to a possible condition of all other appendages, the coxal joint 

 of the first pair of antennae is never absorbed into or fused with the sternal 

 portion or ventral arc of the somite to which it belongs. 



The third pair of appendages consists of the second pair of antennte. 

 These are often very large and powerful organs, frequently adapted as 

 weapons of offence and prehension. They consist of two divisions similar 

 to the first pair, that is, a peduncular and flagelliform part. Of these the 

 peduncular consists of five joints, the flagelliform extremity of a strong, 

 solitary, multiarticulate rod in its most normal condition ; but it very fre- 

 quently varies in form, but never increases in the number of its branches. 



• In the Macrura generally the flageUum is produced, on an average, to 

 about the length of the animal, and is mostly multiarticulate in its character, 

 the small articuli varying in ntimber and length. .Sometimes, as in Scyllarus 



