No. 26.] ARTHROSTRACA OF CONNECTICUT. 189 



under the sides of the posterior part of the carapace. In the 

 Oniscoidea air sacs, or tracheae, adapted for air breathing, are 

 developed in some or all of the pleopods. In Tanais the pleopods 

 are reduced to three pairs ; and in the Asellidae the first pair of 

 pleopods is quite small and the outer rami of the second pair are 

 very large, forming a kind of operculum in the female, and com- 

 plex copulatory organs in the male. In the Oniscidae and 

 Armadillididse the first and second pairs of pleopods are modified 

 to form sexual organs in the males, those of the first pair often 

 being coalesced in the Oniscidse and rudimentary in the females. 



INTERNAL ANATOMY. 



The alimentary canal is a straight tube much as in the 

 Amphipoda. A ccecal appendage corresponding to the rectal 

 gland is lacking. From the union of the stomach and the intestine 

 there extend posteriorly four hepatic cceca which are more or 

 less coiled. 



The heart is an elongated tube suuated to a great extent in 

 the abdomen except in the Tanaioidea, where it occupies the 

 anterior part of the thorax, and in the Asellidae, where it is con- 

 fined to the thorax. The dorsal aorta arises from the anterior end 

 and in the method of branching there is great variation in different 

 species. 



The nervous system and reproductive organs in general are 

 similar to those of the Amphipoda. 



BIOLOGY. 



Like the Amphipoda this group is for the most part marine, 

 but unlike that group there are some forms among the Oniscoidea 

 which have extended their habitat to land and are fully adapted 

 to a terrestrial form of life by the modification of the pleopods. 

 Within New England there is but a single fresh-water form 

 Asellus communis which is widely distributed and very abun- 

 dant. The terrestrial forms include several of practically world- 

 wide occurrence such as Armadillidium vulgare, Porcellio scaber, 

 Oniscus asellus, Metoponorthus pruinosus. 



Of the European forms which are found on the Atlantic coast 

 of North America are Idothea metallica, Cyathura carinata, Mga 

 psora, Cirolana concharum, Idothea marina, Jcera marina, Lepto- 

 chelia savignyi, Tanais cavolinii, Linnoria lignorum. 



