292 



CRUSTACEA. 



exopodite developed as a respiratory plate fringed with setae. The 

 two anterior maxillipedes (Fig. 136, mf\ mf") have developed as 

 biramose swimming limbs. The endopodite of the first maxillipede 

 has four joints, while that of the second maxillipede is more rudi- 

 mentary and usually consists of three short joints. The exopoditea 

 (flagellate branches) have long swimming setae at their ends. The 

 rudiments of the succeeding limbs (third maxillipedes and first five 

 ambulatory limbs) seem entirely wanting in the youngest Zoaea of 

 many Brachyura (e.g., in Pinnixa, W. Faxon), in other cases some 

 or all of them appear as short, truncated outgrowths (Maja, Inachus). 

 The pleopoda are still altogether wanting. The abdomen is generally 

 distinguished by a definite development of spines, the second segment 



A '* 



Fig. 137.— Limbs of an older Zoaea of Portumis (after Glaus). A, first antenna. B, secoi 

 antenna. C, mandible. D, first maxilla. E, second maxilla, en, endopodite ; at, exopodit 

 ex, spinous process ; t, mandibular palp. 



having a pair directed anteriorly, and the three succeeding segments 

 each having a pair directed posteriorly. The telson, as a rule, has a 

 characteristic forked form, and is produced out laterally into a long 

 spine. On the inner side of these two processes on the telson there 

 are, in most cases, three strong setae. 



The form of the Brachyuran Zoaea undergoes, in isolated cases, variations 

 from the type here described, which are chiefly connected with the develop- 

 ment of Hit; spinous processes and the form of the telson. In Gelasimus, for 

 instance, the spinous processes of the dorsal shield are unusually short. In 

 Achaeus, there is no frontal spine, but a short dorsal spine is retained. In 

 Tnachua also the frontal spine is wanting (Claus, No. 8; Gourmet, No. 130) 

 In Maja (Couch) and in Eurynome (Kinahan) all the spinous process.- 



