30 REPORT — 1855. 



consciousness analogous to sound to the auditory nerve, and on this account 

 we shall allude to tlieni under the name of Auditory Cilia. 



Professor Milne-Edwards considers the presence or absence of the se- 

 condary filament or palp as a circumstance of little importance, and affirms 

 that naturally the genus Amphitoe, without this appendage, is extremely near 

 to Gammarus, in which it exists, if they be not in the same genus*; the 

 separation being admitted for the convenience of classification only. 



But from this our experience compels us to differ. The two filaments, how- 

 ever unequal, homologize with those in the higher order, where stmietimes a 

 third is added, two of which are, to the extent of our present knowledge, 

 always constant. We therefore can but view the presence or absence of 

 this palp, however rudimentary the form in which it may exist, as de- 

 monstrative of some change in the habits or condition of the animal, which 

 must be accompanied by structural alteration of a more or less important 

 character. It must therefore show a separation between animals that vary in 

 some essential conditions, even though not very visible features. 



Thus it will be found upon a close examination that Amphitoe is separated 

 from Gammarus by important essential qualities (which will be described 

 with the animals in our forthcoming work on this subject in conjunction 

 with Mr. Westwood). Here it is sufficient to observe, that the habits of 

 Amphitoe, as well as its structure, are closely allied to tliose of the genus 

 Podocerus, and that they both exist in a division (Nidi/ica) of the family 

 CorophiidcB, which division we have thought desirable to construct, that 

 those Amphipoda which live in nests of their own construction may be 

 separated from those which live in tubes, or burrow, such as Cerapus and 

 Corophium. 



The second or external pair of antenncB. — These organs appear to us to be 

 the most anterior appendages, which are supported in- the Amphipoda upon 

 a segment that is present, and which forms almost the entire cephalic region. 



One of these antennae consists typically in the order of a peduncle and a 

 solitary filament. The peduncle consists of five articulations. In some, as 

 the Macroura, there is attached a moveable scale ; and in others, as the Ano- 

 moura, a spine exists on the basal portion of the antenna : these appear 

 both to be represented in the larva of the Brachyura. and at an early period 

 of this stage are more important than the principal appendage of the an- 

 tenna itself. These secondary parts are absent in the Amphipoda. 



The first or basal joints of this organ in the Brachyura are very generally 

 fused together, and with the nearest approximating part of the calcareous 

 skeleton of the animal ; this fusion is sometimes so perfect, that no mark of 

 distinction is apparent to distinguish the antenna from the body of the 

 animal : this is particularly correct of the Leptopodiadcc. But this close 

 union between the parts of the antenna and the body of the animal lessens 

 with the degradation of the creature, until we find the five articulations 

 separate from each other and distinct from the animal. This is the case in 

 the Macroura as well as Amphipoda. 



But even in this order, Amphipoda, in many species it is with difficulty the 

 demarcation between the two first or basal articulations can be made out, so 

 intimately do they appear to be connected together. From the first of these 

 a strong tooth or spine is commonly developed, in some more importantly 

 than in others ; this denticle is the external portion of the olfactory organ, 

 and homologizes with the olfactory tubercle (auditory of M. Milne-Edwards, 

 Von Siebold, &c.), which is situated on the basal portion of the antenna in 

 the PodopJUhalma. 



* Histoire des Crustaces, vol. iii. p. 28. 



