636 CRUSTACEA. 



tion is performed by the general surface of the body ; in. a few species 

 only, certain abdominal appendages appear to take part. 



The existence or non-existence of thoracic branchial appendages 

 marks the higher and lower grades of Mysidea, and this characteristic 

 is, therefore, a proper basis for family distinctions. Moreover, the aber- 

 rant structure in Lucifer, removes these species widely from the other 

 Mysidea, the great length of the antennary segment being connected 

 with a very short thorax and simple thoracic legs, without a natatory 

 appendage, or only an obsolescent palpus. 



There are, hence, three prominent groups or families among the 

 Mysidea : 



Fam. I. EUPHAUSID^E. Cephalothoracis Caridoideus. Pedes thoracici 

 bifidi, appendicibus branchialibus instructi. 



Fam. II. MYSID^E. Cephalothoracis Caridoideus. Pedes thoracici 

 bifidi, appendicibus branchialibus carentes. 



Fam. III. LUCIFERIDJE. Segmentum antennale valde elongatum, 

 carapace per suturam fere discretum. Pedes simplices. 



The Luciferidae are the transition species connecting the Mysidea 

 with the Amphionidea, the antennary segment in the latter being 

 expanded to an unusual size, and separated by some appearance of a 

 suture from the following part of the cephalothorax. 



The Euphausidae and Luciferidaa, as far as at present investigated, 

 appear to form but a single group each. The species of the former, 

 in all cases examined by the writer, have a naked setaa either side of 

 the extremity of the caudal segment, which often gives it a barbed 

 extremity, with the barbs reversed. This character has not been ob- 

 served in other Mysidea. 



The Mysidaa include three groups : 



The first (Oi/nthince) with abdominal branchial appendages, in the 

 form of a coiled membrane or plate, and the inner antennae two- 

 branched. 



The second (Mysitwe), without branchial appendages, but with the 

 inner antennas two-branched. 



