CHAPTER X 



THE CUMACEA 



Order Cumacea, Kroyer (1846). 



Definition. — Peracarida in which the carapace coalesces dorsally 

 with the first three or four thoracic somites, overhangs on each 

 side to enclose a branchial cavity, and is produced in front into two 

 plates Avhich usually meet each other above in front of the head to 

 form a pseudorostrum ; the telson may be coalesced with the last 

 somite ; the eyes are generally coalesced into a single organ set on 

 an immovable process of the head ; the antennules may be 

 biramous ; the antennae have no exopodite ; some of the thoracic 

 limbs have natatory exopodites ; the first three pairs are modified 

 as maxillipeds ; the first pair have an epipodite generally pro- 

 vided with branchial lobules and an exopodite forming a respiratory 

 siphon ; the pleopods are absent in the female and often reduced 

 in the male ; the uropods are styliform ; the young leave the brood- 

 pouch before the appearance of the last pair of thoracic limbs. 



Historical. — The first described Cumacean Avas the Oniscus 

 scorpioides of Lepechin (1779), and other species were described by 

 Montagu (1804), Say (1818), and H. Milne-Edwards (1828). The 

 last-named author established the genus Cuma, from which the 

 name of the order is derived, but he later regarded this as being a 

 larval decapod, and he maintained his opinion of the larval nature 

 of the group as late as 1858, although Kroyer had described 

 ovigerous females in 1841 and his discovery had been confirmed 

 by H. Cxoodsir and others. While Spence Bate, Norman, Lilljeborg, 

 Hansen, Dohrn, and others have contributed descriptions of species 

 and observations on structure and development, by far the greater 

 part of our present knowledge of the group is based on the elaborate 

 and beautiful memoirs of G. 0. Sais. 



Morphology. 



The general shape of the Cumacea is usually very characteristic, 

 owing to the sharp distinction between the inflated cephalothoracic 



183 



