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HYDRACARINA: THE GENUS EYLAIS LATR. 



By Charles D. Soar, F.L.S., F.R.M.S., and W. Williamson, 

 F.R.S.E., F.L.S. 



(Read January Uth, 1920.) 



Plate, 3. 



The genus Eylais * was established by Latreille in 1796, and is 

 notable by reason of the structure of the capitulum and epimera 

 and of the eye plate, which consists of a pair of eye capsules 

 united by an intercapsular bridge as well as by its pronounced 

 disposition to pass into varietal forms. Several widely distributed 

 forms, though usually recorded as distinct, have been suspected for 

 long to be either merely varieties or stages in the development of 

 one species. The difficulty of obtaining suitable material for 

 working out the life-history adds greatly to the complexity of the 

 problem, even in normal times. When one recollects C. L. Koch's 

 reputation as a species maker, there is reason to be thankful that 

 the intercapsular bridge was too small a feature to arrest his 

 attention, otherwise succeeding workers might well be baffled in 

 the endeavour to reduce Koch's species to their proper limit. 



Apart from its varietal disposition the genus has several well- 

 defined characteristics. The capitulum is short, and on its 

 flattened ventral surface it bears the sucker-like mouth, bor- 

 dered by a fringe of hairs. The basal portions of the mandibles 

 form a pseudo-capitulum. The paired eyes are in capsules, 

 which are united by a bridge varying with the degree of separation 

 between the capsules. In one or two species the capsules may be 

 separated and the bridge wanting. Considerable prominence, 



* An old form of the name omits the y. Piersig, apparently founding 

 on Agassiz, changed the name to Eulais, Later Koenike, vmable to 

 find any support for the idea of Greek origin, restored the original name 

 Eylais. 



In the glycerine medium used for other genera, Eylais does not 

 behave well, as frequently specimens are fo\md to have become unsuit- 

 able for useful work. A thymol solution introduced by Koenike 

 appears to have more satisfactory results. 



JouRN. Q. M. C, Series II.— No. 86. 8 



