42 report— 1877. 



with other parts, the normal character of this pair of antennae is that of a 

 peduncle of five joints and a terminal flagcllum, variahle in length, and, with 

 but few exceptions, consisting of a solitary branch. 



The centre or third joint of the pedunclo in some orders, as the Macrura, 

 invariably carries a squamose or scale-like plate ; this varies in size and a 

 little in form, but disappears in the higher and lower orders, and again 

 reappears in the genus Apseiides among the Isopoda. This squamiform 

 plate is, I believe, homotypical with the secondary branches of the flagcllum 

 of the first pair of antenna?, therefore a correlative of the same. 



In the Brachyura the first three joints of the base or peduncle of this 

 antenna are more or less perfectly fused with the dermal tissues of the 

 cephalon. In some, as in most of the triangular genera, as Stenorhynchus, 

 Pisa, &c, the line of separation between the somite and the appendage is 

 indistinguishable in the adult. This is also more or less the case in several 

 forms of Brachyura, and makes a ready and safe key to generic distinction. 

 In all these forms the flagellum is reduced to a feeble condition, and becomes 

 almost rudimentary in those of terrestrial habits. 



In the Macrura the genus Scyllarus and its near allies have the flagcllum 

 transformed into a broad plate or scale ; but in Crustacea generally this 

 appendage is niultiarticulate, robust, and long. In some genera, as in 

 Palinurus, it is used as a weapon of offence as well as for other requirements. 



In the Amphipoda this antenna is simple and normally well defined, the 

 five joints of the peduncle and the flagellum being separate and distinct, and 

 the whole appendage robust and long, the two parts (■/. e. the flagellum and 

 peduncle) being generally subcqual. But in those genera that exhibit a 

 variation, the higher class has the peduncle the more important, as in the Or- 

 chestidce, whereas in the male of Cerapus, as compared with the female of the 

 same and the Hyperidce in general, it is less so. Almost universally the flagcllum 

 is delicately multiarticulate, varying from a small number of articuli, as in 

 Corophium, to an innumerable quantity, as in some species of Bath/poreia. In 

 the genus Clydonid the flagellum consists of a long, rigid, non-articulate spine. 

 Among the Hyperidce the antenna is considerably impoverished, and in many 

 genera it is rudimentary, while in Phrosina it appears to be absent. 



In the parasitic Amphipods, such as Cyamus, as compared with the pre- 

 ceding antenna, the second is well developed and important, but not so much 

 so as in the organs of the normal Amphipods. 



In the Isopods this appendage is seldom very important, being largest it 

 the terrestrial forms, as Ligia, Oniscits, &c, and in some aberrant genera, 

 like Arcturus &c. 



By most carcinologists this pair of antenna? is considered to be tbo scat 

 of an organ of sense. It has been worked out and displayed by M. Milne- 

 Edwards, in his ' Histoire Naturelle des Crustacea,' vol. i. p. 124, pi. 12. 

 figs. 9, 10, both in Homarus and Maia. He argues that the structure 

 demonstrates the auditory character of this organ, as we have shown in 

 investigating the evidence relative to the functional properties of the pre- 

 ceding pair of antennae. 



As Milne-Edwards, in his 'Histoire jSTaturcllc des Crustaccs,' vol. i. p. 124, 

 1840, suite a Buffon, bases his opinion on the character of the structure of 

 the organ at the base of the second pair of antennae, it is but just that his 

 reasons should be communicated as literally as translation will convenient!)- 

 admit. He says : — " In Maia and other short-tailed Crustacea there is a 

 very curious operculum. M. Audouin and I have observed that it is con- 

 nected with a moderately large osseous plate, which separates from it at 



