MARINE ISOPODA OF NEW ENGLAND, ETC. 303 



The body is said to be depressed, or flattened from above, in distinction 

 from the form usually seen in the Amphipoda, where it is in general flat- 

 tened from side to side. An important exception to the ordinary mode 

 of respiration occurs in the Tanaidce, as has already been mentioned. 

 In this family respiration takes place in two lateral cavities, situated 

 beneath the integument of a large cephalothoracic shield, covering the 

 head and first thoracic segment. In general, as the name of the order 

 indicates, the legs are similar in structure and function throughout, as 

 in the " sow-bug," but may differ considerably, as in the Arcfuridw, the 

 Miamopsidce, and the Tanaida\ 



The arrangement of the families in the present paper can only be re- 

 garded as tentative, and no higher grouping will be attempted further 

 than to indicate briefly the relationships of a few of the families to each 

 other. 



The Omscidce may, on account of their aerial respiration, be regarded 

 as standing quite distinct from the remaining families, and should, per- 

 haps, be further divided as proposed by Kinahan. As they do not, how- 

 ever, come within the proper scope of this article, I have not attempted 

 to subdivide the family. The Bopyridce have been placed near the Onis- 

 cidce in deference to the opinions of Dr. Fritz Miiller. Having made no 

 study of this family myself I do not express any opinion as to the pro- 

 priety of separating it so widely from the Cymothoidw, with which it has 

 usually been associated. The Asellidce and Mmmopsidce are closely allied 

 to each other. The Idoteid^ and Arcturidce form a group distinguished 

 especially by their operculiform uropods. The above families correspond 

 nearly with the " marcheurs" or walking Isopoda of Edwards, and more 

 nearly with the " gehende Asseln " of Miiller. They usually have the 

 antennulsB much less developed than the antennae, and the uropods ter- 

 minal or inferior, that is, attached to the end of the last segment, or in 

 the last two families to its inferior surface. 



The SpJiceromidce and Limnoriidce are closely allied, and perhaps ought 

 hardly to be kept separate as families. The Cirolanidce, ^gidce, and 

 Cymothoidw form another group embracing a wide diversity of forms, 

 from the active predatory Cirolana to the sedentary and distorted Livo- 

 neca, and yet apparently connected by easy gradations. The remaining 

 families are generally regarded as aberrant, and form the "Isopoda 

 aberrantia " of Bate and Westwood. They do not present any very evi- 

 dent relationships with the preceding. Of these the Anthuridoc have 

 usually been associated with the Idoteidm or the Arcturidce, or with 

 both. Except an elongated form, however, they do not appear to have 

 rnmch in common with either of these families. According to Dohrn's 

 observations they are related to the Gnathiidce in the structure of the 

 Incubatory pouch. The Gnathiidce have the head united with the first 

 thoracic segment, as in the Tanaid^, but this last family is widely sep- 

 arated from the others, and doubtless ought to be regarded as forming 

 a distinct suborder, according to the views of Dr. Fritz Muller. 



