THE EUPHAUSIACEA 



plate Avhich in the adult has been regarded as the exopod, but which 

 is in reality a process of the first segment of the limb. A very 

 remarkable feature in the development of the paired eyes has been 

 found by Sars in the case of Nematoscehs. Here, a transitory larval 

 eye consisting of seven ommatidia is formed at the apex of the eye- 

 stalk and is later pushed to the outer side by the development of 

 the permanent visual organ. Though this transitory organ is stated 

 to disappear in the adult, it seems possible that it may really persist 

 as the luminous organ of the eye-stalk, especially since in other 

 genera {NycUphanes, Euphausia) the luminous organ appears before 

 the eye itself. The heart is visible already in the later metanauplius- 

 stages. The liver at first consists of three caecal tubes on each side. 



Remarks on Habits, etc. 



The Euphausiacea are characteristically pelagic animals, forming 

 jDart of the surface-plankton of the ocean and descending to con- 

 siderable depths. They are generally remarkably transparent, but 

 Bentheuphmisia, Avhich appears to be a true deep-sea form (1000- 

 1800 fathoms), is said to be "quite opaque and of a similar vivid 

 red colour to that of most other true deep-sea crustaceans." The 

 adult size of most Euphausiacea lies between 10 and 40 mm.; a 

 species of Thysanopocht reaches 55 mm. in length. 



No fossil Euphausiacea are known. 



Affinities and Classification. 



The differences in structure which justify the separation of the 

 Euphausiacea from the Mysidacea have already been insisted on. 

 A certain degree of resemblance in general facies is sufficiently 

 accounted for, on the one hand, by the approximation of the basal 

 members of the Peracaridan and Eucaridan lines of descent to the 

 common caridoid stock of the Eumalacostraca ; and on the other, by 

 the similarity in habitat betAveen the Euphausiacea and many of 

 Mysidacea. The resemblances betAveen the members of the present 

 Order and some of the lower Decapods, especially the Penaeidea, 

 are of much greater importance. The complex copulatory armature 

 of the first pleopods has a general resemblance to that of the 

 Penaeidea, the larval development of the two groups is closely 

 parallel, and the presence in some Sergestidae of phosphorescent 

 organs lesembling, though differing in details from, those of the 

 Euphausiacea may also be an indication of affinity. 



At present, the Euphausiacea are regarded as constituting only 

 a sinojle familv. 



