CHAPTER XIV 



THE EUPHAUSIACEA 



Order Euphausiacea, Boas (1883). 



Definition. Eucarida which retain the primitive caridoid facies ; 

 the exopodite of the maxilla is small ; none of the thoracic limbs 

 are specialised as maxillipeds ; branchiae in a single series, attached 

 to the coxopodites of the thoracic limbs (podobranchiae) ; young 

 hatched in the nauplius-stage. 



Historical. The genus Thysanopoda was placed by H. Milne- 

 Edwards in his tribe of " Mysiens," and most subsequent writers 

 have retained the association of Euphausiacea and Mysidacea in the 

 group " Schizopoda." As already mentioned, Boas was the first to 

 separate the two orders. Some of the later larval stages were 

 described as distinct genera by Dana before their true nature was 

 pointed out by Glaus. Metschnikoff's discovery of the nauplius- 

 stage of Euphausia (1869) drew special attention to the larval his- 

 tory of the group. The most complete account of the structure, 

 development, and classification of the Order is that given by G. 0. 

 Sars in his " Challenger " Report. The phosphorescence of certain 

 Euphausians was first observed by J. V. Thompson ; the light-pro- 

 ducing organs have been investigated by several zoologists, the 

 most detailed account being that given by Chun. 



MORPHOLOGY. 



The general form of the body (Fig. 139) is completely caridoid. 

 The carapace coalesces dorsally with all the thoracic somites, and 

 is produced in front as a rostrum which is never of great length. 



The telson (Fig. 142, C) has a pair of large movably articulated 

 spines (s) near the tip. Sars has shown that these are formed by 

 enlargement of one out of several pairs of marginal setae which are 

 present in the larvae, and it is therefore impossible to regard them 

 as representing the caudal furca of the Leptostraca. 



Appendages. The antennules are always biramous, and some- 



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