70 



NATURAL HISTORY OF ARTHROPODS. 



FIG. 89. Leptochelia alyicola, enlarged twenty times. 



" sow-bugs," live on the land. The Isopocla are usually divided into two groups, 

 named, respectively, Ambulatoria and Natatoria, according to their walking or swim- 

 ming habits, with which are correlated many anatomical features. We will here, how- 

 ever, omit these two larger divisions, and proceed at once to the discussion of the 

 families. 



The first of these families, the TAJSTAID^E, is of doubtful position, but, according to 

 Fritz Miiller (whose opinions are seconded by other aiithorities) they probably repre- 

 sent an ancestral type of Isopoda. 

 They differ from all other Isopoda 

 in having the respiratory organs be- 

 neath the head and first thoracic se- 



O 



ment, the eyes, when present, upon 

 jointed stalks, while the abdominal 

 appendages have no respiratory func- 

 tion, and the first pair of ambulatory feet are chelate. In their embryology the 



Tanaidae are said to present points of resemblance to Asellus, Lygia, and Cnma. 



But two genera, Tanais and Leptochelia, are represented, 



so far as is known, upon our shores. 



In the GNATIIIID^E we find a type more allied to the 



normal isopod. The thorax apparently consists of five 



segments, and only five pairs of limbs possess the normal 



form. In the typical genus Gnathia (Praniza and Anceus 



of authors) there is a great difference between the males 



and females, so great, in fact, that different sub- families 



have been erected for the two sexes of the same species. 



The larva? are more nearly like the normal isopod than are 



the adult. A single species, G. cerina, is found on the 



New England coast. 



Passing the ANTHURID^E, a small group of slender 



forms, the next families to be noticed are the CYMOTII- 



ERID.E, ./EGID.E, and CIROLANID.E, which embrace most 



of the parasitic forms of this order. Yet the parasitic 



habits have not produced such a profound effect upon the struc- 

 ture as among the root-barnacles and the Siphonantia, already de- 

 scribed. From their habits the term "fish lice" is 

 eminently appropriate for them, as they live at- 

 tached to the integument or within the mouth of 

 fishes. Usually these three families are considered 

 as one, under the name .^EgidaB. They all agree in 

 having all of the segments behind the head dis- 

 tinct, and in the lack of an operculum closing in 

 the respiratory cavity. 



Among the more interesting forms belonging 

 to this family may be mentioned jSSga, psora, the 

 " salve-bug " of the fishermen, which is frequently 



found attached by its sharp claws to the cod and halibut, and Cymothoa 



prcpgustator, which occurs attached to the roof of the mouth of the 



menhaden. Mr. J. F. Bullar has shown that in certain genera (Cymo- psora, skive-bug 



Fn;. 00. Gnathia, male. 



FIG. 91. Gnatliia, 

 female. 



