No. 26.] ARTHROSTRACA OF CONNECTICUT. I3 



CLASSIFICATION OF THE ARTHROSTRACA. 



The Amphipoda and Isopoda together constitute the tribe or 

 legion of the Malacostracous Crustacea known as the Arthros- 

 traca. The Arthrostraca are characterized by the absence of a 

 carapace, the head being fused with never more than one thoracic 

 segment, the presence of only seven free thoracic segments, a 

 single pair of maxillipeds, sessile eyes, and thoracic limbs which 

 are uniramous throughout life. The two groups in question 

 are sometimes referred to as the sessile-eyed Crustacea (Edrioph- 

 thalma) to distinguish them from the other Malacostraca in 

 which the eyes are mounted on movable stalks, the stalk-eyed 

 Crustacea (Podophthalma). 



The Amphipoda may be defined as Arthrostraca in which the 

 body is generally strongly compressed from side to side ; and with 

 the abdomen consisting typically of six segments and a telson. The 

 gills are borne on the inner side of the proximal joints of the 

 thoracic legs, and, corresponding to the position of the gills, the 

 heart lies in the anterior part of the body. 



The Isopoda may be defined as Arthrostraca in which the body 

 is generally strongly flattened dorso-ventrally ; with the abdomen 

 composed of six segments, which may be partly or wholly fused 

 together, and the terminal one which is referred to as the 

 telson and is large and shield-like but rarely free ; the respiratory 

 organs are borne on abdominal segments and consequently the 

 heart has an abdominal position. 



The most important characteristics which distinguish the two 

 groups are: (i) The general form of the body which is flattened 

 in the Isopoda, and compressed laterally in the Amphipoda ; and 

 (2) the respiratory organs, which are usually modified pleopods 

 in the Isopoda^ and always appendages of the thoracic limbs in 

 the Amphipoda. In the former group the five anterior abdominal 

 appendages are usually broad plates which are all more or less 

 similar in shape and size. In the Amphipoda the first three 

 pairs are similar, being long, narrow appendages adapted for 

 swimming while the last three pairs are short and stiff and are 

 adapted for leaping. 



The classification of the Amphipoda into families is not in 

 an entirely satisfactory condition. The group is very diversified 

 and there is a great difference of opinion regarding its proper 



