Order AMPHIPODA. 



The animals that constitute this Order of Crustacea exhibit the 

 characters of the Class perhaps more perfectly typical than can be 

 found in either of the others ; for the type of a class or family ia 

 more commonly to be distinguished in its centre than at either ex- 

 tremity of its development. 



The three dh-isions of the animal are distinctly marked. 



The cephalon, the pereion, and the pleon never encroach upon 

 one another^ and the appendages assume a characteristic form in. 

 each. 



The cephalon is never developed beyond its normal range, and 

 appeai-s to be scarcely larger than a single segment. It is never 

 produced into a carapax. 



The pereion has seven segments, all dorsally perfect and seve- 

 rally distinct. To the normal condition there are few exceptions. 

 In the genus Dulichia the sixth and seventh segments are fused 

 together; in Phrosina the first and second are so incorporated, 

 whilst in Rhahdosoma one segment is absent. 



The pleon has seven segments (the last rudimentary), severally 

 distinct, and is capable of being doubled bcueath the pereion. To 

 this normal condition there are exceptions. 



In Chelura the three penultimate segments are fused into one ; 

 in CyrtopMum and Dulichia the one preceding the telson is want- 

 ing ; and in Caprella and the allied genera all the segments of the 

 pleon are more or less imperfect or wanting. 



The appendages attached to the cephalon are more or less con- 

 nected with the organs of sense. 



The eyes (a) are compound and sessile, and lodged between the 

 bases of the two pairs of antennaj. The tissue Avhich covers them 

 is not divided into facets. 



In Ampelisca the eyes appear like four simple organs rather than 

 compound. In Westwoodia ccecida the lenses of the organs are 

 not appreciable. In some genera of the Hypeeina the tissue over 

 the organs of sight exhibits traces of being marked out into 

 facets. 



The first or upper pair of antennae (h) consist of a peduncle of 

 three articidations, and a terminal multiarticulate filament supplied 

 with auditory cilia. Occasionally there is a second appendage, 

 generally nidimcntary, seldom impoi-tant, but never supplied with 

 auditory cilia : tliis secondary appendage exhibits its maximum ap- 

 pearance in the Phoxides. 



The second or lower pair of antonnaj (c) possess a peduncle of 

 five articulations, and a niultiarticulute flagellura. Tlie first two 

 articulations of the peduncle, and sometimes the tbiid, as in Talitms 



B 



