ON THE BRITISH EDRIOPHTHALMA. 29 



In the genus Tetromatus, which, we believe, is now for the first time added 

 to our knowledge, there are four simple eyes, two upon each side of the head, 

 instead of one made up of many facets, as is usually the form of the organ 

 in this class of animals. But this seeming anomaly appears not to be with- 

 out explanation. 



In the young of the Amphipoda the number of facets is fewer in the eye 

 than in the adult; the number of the lenses therefore increases with growth. 

 In the genus Gammarus the early numbers are eight or ten, whilst those of 

 the adult are from forty to fifty. If we suppose tliat in Tetromatus there 

 were but two crystalline lenses developed in the larva, a consequent 

 arrest of development at this particular stage would limit tlie number in the 

 adult to those already present in the larva, and which therefore, we think, 

 must be looked upon rather as two distant lenses of the same eye, than as 

 distinct organs of vision, although to external observation they assume the 

 appearance of two separate eyes (Plate Xllf. fig. 8). The coloured cornea 

 is very distant from the lenses. 



In this genus the crystalline lens is developed in the integumentary struc- 

 ture of which it forms a part ; in this arrangement the condition of the eye 

 differs from that of any other among the Amphipoda. Close observation 

 may detect a lessened approximation of like condition in Anonyx HolboUi, 

 but there only a semi-transparency, like a single small lens, exists. 



The sessile character of the eyes in this order appears chiefly to rest on 

 the pedunculated feature being absent rather than in any definite alteration 

 of the eye itself, and by no means is it to be considered as evidence of organs 

 of vision indicative of a lower class of animal. This we think is easily 

 demonstrated by the fact, that in all the DiastylidcB the eyes are sessile and 

 converge into a single organ ; this is the case also with some of the Ento- 

 Tnostraca, whilst, on the other hand, the genus Tanuis among the Edrro- 

 phthalma, and Artemia among the Entomostraca, have the eyes supported 

 on footstalks in a manner corresponding with the higher types. 



The internal or first antennoe. — These organs are invariably constant in 

 the order Amphipoda, although in the genera of Orchestia, Talorchestia and 

 Talitrus, they are so unimportant as to be little more than rudimentary 

 appendages. They belong to the second normal segment, which in the^m- 

 phipoda we believe not to be developed, or, if present, fused so completely 

 witii the next succeeding, as not to be distinguished from it. 



The anterior antennae typically consist in all Crustacea of a peduncle 

 formed of three articulations, all of which are present in the Amphipoda ; and 

 a filamentary appendage more or less extensively developed, and one or two 

 secondary filaments of greater or less importance, of which latter in the 

 Amphipoda there is never more than one, and that is generally rudimentary, 

 often obsolete, and perhaps move frequently absent than present. But this 

 secondary appendage appears to fulfil but an unimportant office even in the 

 higher orders, whilst in the Atnphipoda it consists of but a few short arti- 

 culated joints furnislied at the extremity of each with a few hairs of a form 

 similar to others peculiar to the species. 



It therefore differs from the principal filament or tige, as it is named by 

 M.-Edwards, which, except in the subfamily of Pontoporeides, is developed 

 to a much greater extent, and in addition to the simple hairs, is furnished 

 with a considerable number of membranaceous cilia, which appear to be 

 peculiar to this organ in Crustacea. The forms of these cilia vary in certain 

 species, and will be more particularly described when it becomes necessary 

 to consider the especial senses of the Amphipoda. We shall only here 

 remark, that they appear to us to be active agents in communicating a 



