No. 26.] ARTHROSTRACA OF CONNECTICUT. 25 



these appendages, there may also be the chelate type, in which 

 the propodus i ■; prolonged at the posterior margin distally so that 

 the dactyl strikes against the anterior margin of this, as in the 

 pincers of the lobster or crayfish. The second gnathopod is 

 chelate in Lysianassa and in the females of Talorchestia and 

 Orchestia. In a few species the gnathopods are found to be 

 complexly subchelate, in that a process of the carpus becomes 

 involved in grasping. Thus in Microdeutopus, the first gnathopod 

 of the male, and in Ericthonius and Cerapus, the second pair is 

 complexly subchelate. Among the Caprellidea the second 

 gnathopods are, as a rule, inserted near the front margin, or at 

 least in front of the middle of their segment, although in the male 

 Caprella the insertion is very near the middle. 



In a number of families the first and second pereiopods 

 possess peculiar glands situated for the most part in the basal 

 joints, but often extending into the distal joints as well, and 

 opening to the exterior at the apex of the dactyl, which is then 

 slightly truncate, or at a point just proximal to the tip. Such 

 forms as Ampelisca, Microdeutopus, the Amphithoidse, Jassidae, 

 and Corophiidas, secrete by means of these glands a sticky sub- 

 stance which hardens quickly to form a thread. By means of this 

 secretion a tube is formed for concealment as will be described 

 below (p. 39). 



In the native Caprellidea the first and second pereiopods are 

 entirely wanting, but the three posterior pairs are well developed 

 and subchelate. They increase in size from the anterior pair 

 to the posterior. 



Abdominal Appendages. 

 The abdomen bears two quite distinct types of appendages, 

 and for this reason is frequently differentiated into an anterior 

 pleon of three segments and a posterior urosome. Each segment 

 of the pleon bears a pair of pleopods adapted for swimming, 

 each of which consists of an unjointed basal joint or peduncle 

 with two flexible, multiarticulate rami distally. These are pro- 

 vided marginally with long plumose setas. The two peduncles 

 are held together by a series of hooks, called coupling spines, 

 which are situated near the distal end. In Corophium the pedun- 

 cle is broadly expanded medially but in other forms it is more 

 nearly cylindrical in form. 



