CHAPTER IX 



THE IMYSIDACEA 



Order Mysidacea, Boas (1883). 



Definition. — Peracarida whicli retain more or less completely the 

 primitive caridoid facies ; the carapace extends over the greater 

 part of the thoracic region, hut does not coalesce dorsally with more 

 than three of the thoracic somites ; the eyes, when present, are 

 movably pedunculate ; the antennules are biramous ; the antennae 

 have usually a large scale-like exopodite ; the thoracic limbs (except 

 sometimes the first and second pairs) have natatory exopodites ; the 

 first and sometimes also the second pair are modified as maxillipeds ; 

 a lamellar epipodite is present on the first pair ; ramified branchiae 

 may be attached to the body-wall close to the bases of the thoracic 

 limbs ; the pleopods are often reduced ; the uropods are lamellar, 

 forming a tail-fan ; the young leave the brood-pouch provided with 

 all the appendages of the adult. 



Historical. — The group Schizopoda, established by Latreille in 

 1817, and long approximated to the Stomatopoda on the authority 

 of H. Milne -Edwards, finds a place in most modern systems of 

 classification, as comprising, after exclusion of many larval Decapods 

 formerly referred to it, the forms here treated of together with the 

 Euphausiidae. Boas, in 1883, however, discarded this grouping, 

 and established the two orders Mysidacea and Euphausiacea, point- 

 ing out that they were by no means closely related, and this view 

 has been also advocated by Hansen. Our present knowledge of the 

 structure and classification of the ]\Iysidacea is very largely due to 

 the work of (I. 0. Sars. 



Morphology. 



From five to seven of the thoracic somites are distinct, and the 

 last two or three of these may be left uncovered by the carapace 

 on the dorsal side. The carapace may be produced in front as a 

 short rostrum, and there is usually a transverse groove or " cervical 

 sulcus" (Fig. 99, c.s) on the dorsal surface in the region of the 



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