86 REPORT — 1876. 



(fig. 16), it consists of a single disk -like plate. But the greatest tendency to 

 variation in form exists in the Amphipod and Isopod Crustacea. In some 

 of these it reaches to a very considerable length and is multiarticulate, hut 

 in others it is reduced sometimes in length, sometimes in form. In Talitrus 

 it is reduced without alteration of character to a very small size ; so it is in 

 Hyperia ; but while in the former it stands on a long and powerful peduncle, 

 in the latter the peduncle is short and feeble. In Ohehira the flagellum is 

 broad, fiat, and uniarticulate, and fringed with a dense mass of soft hairs. 

 In Podocerus and a few closely allied genera the flagellum is formed of one 

 or two large articuli or joints, and the hairs are reduced in number but 

 increased in strength, and become hook-like spines. In Coroj)Mimi the 

 whole antenna bears a near resemblance to a true walking-appendage, and 

 is no doubt used to assist in progression, as is mostly the case with Crus- 

 tacea that inhabit tubes and hollows of their own excavation or building. 



The peduncle of this antenna is invariably formed of five joints. These 

 are :— 



The first, for which Professor Milne-Edwards has suggested, in the memoir 

 quoted, the name of coxocerite. This contains within it an organ of sense 

 which Milne-Edwards believes to be connected with that of hearing ; 

 but I think there will be little difficulty, when reporting on the internal 

 anatomy, in showing that it is connected with the olfactory sense. In 

 the Amphipoda and Isopoda, with but few exceptions, such as Talitrus, 

 OrcJiesiia, &c., the first joint is free ; but so it is in many of the Macrurous 

 forms, such as Astacus, Homarus, &c. But in Palinurus it is strongly built 

 into and fused with the ventral arc of the fourth or next approximating 

 somite. These parts are stOl more closely associated in the Brachyurous 

 form, so that it is difficult to determine where the antennae end and the 

 region named by Latreille the epistome commences. 



The second joint, named by Milne-Edwards the hasocerite, is generally 

 short and supports at its extremity a movable squamiform appendage, to 

 which the same carcinologist has given the name of sccqJiocerite. This 

 apx^endage is coiastant in all Macrurous forms of Crustacea. It appears to 

 be .wanting in the genus Palimirus only ; but even here it is represented, as 

 I had the opportunity of showing, in the Eeport on " The Marine Fauna of 

 Devon and Cornwall," by a figure of it incorporated in the integument of the 

 succeeding joint, as if it were absorbed by pressure against it. 



This appendage (scaphocerite) does not exist in any of the forms higher or 

 lower than the Macrura, except Pontki (PI. II. fig. 18) in the Entomostracous 

 forms, and that peculiarly interesting little Isopod A2')seudes, in which genus 

 we find a small squamiform plate resembling and probably homologous with it. 



The third joint the above author has named the ischiocerite, and the tAVo 

 following the mesocerite and the carpocerite, while the multiarticulate 

 fiagellum, which corresponds "to the penultimate joint of the thoracic 

 member," he calls the procerite. It is rather a curious oversight that, while 

 Milne-Edwards has been most particular in identifying the several parts of 

 the second antennge by an especial name, he has omitted to give any to those 

 of the first pair of antennae, the three joints of the peduncle of which are 

 homotypical of the coxocerite, the basocerite, and the ischiocerite of the 

 second pair of antennae; but the flagellum, instead of being homotypical of 

 the procerite, represents the mesocerite and the successive articulations. 

 % In the Macrura generally the joints of the peduncle are distinctly separated 

 from one another ; but in some of the higher forms, such as Astaciis, Homarus, 

 and Palinurus, they exhibit a tendency to crowd and coalesce with each other. 



