on some new JEntomostraca. 9 



must be confessed that future researches may perhaps prove tliat 

 the two sexes of one species have been separately described, for 

 the generic descriptions are founded on characters which differ in 

 the two sexes ; and it is impossible to distinguish in all cases a 

 female Undina from a Calanus, or a female Pontella from an 

 Acarlia. 



To avoid if possible this source of error, I h-^we been very care- 

 ful in describing those parts which are generally considered to be 

 alike in both sexes, as, for instance, the organs of the mouth, the 

 natatory legs and the shape of the cephalothorax, and 1 do not 

 think any difficulty will be found in joining the two sexes when 

 both are known ; at the same time, it is not certain that the 

 above-mentioned organs are alike in the two sexes of the same 

 species. 



The characters upoii which species are founded need perhaps 

 some remark, for the hairs are in most cases so variable that it is 

 rather startling to find them relied on in the Entomostraca as 

 affording excellent specific characters. However, the examination 

 of thousands of specimens has convinced Professor Dana that 

 such is the case, and the form and position of the hairs, and 

 especially of those on the terminal portion of the anterior antennae, 

 are as useful in the study of Entomostraca as are the teeth in that 

 of Mammalia. 



The normal number of hairs on each segment of the antennae of — 

 for instance — a CaUinus is four, one on the middle of the anterior 

 margin, and at the apex, two anterior and one posterior. The 

 apical segment appears to offer an exception to this rule, but in 

 such cases the apical segment itself is either very minute or per- 

 haps has disappeared, so that the hairs belonging to two segments 

 are apparently collected on one. In confirmation of this, compare 

 the antennae of Diaptomus abdominalis or Calanus inconspicuus 

 with that of C. brevicornis. C. penicillatus is remarkable for hav- 

 ing a pencil of hairs at the end of the anterior antenna. The 

 antennae are unequal, the longer one having eighteen, the shorter 

 only thirteen segments, and I consider the terminal tuit to be the 

 hairs which remain, although the segments to which they normally 

 belong have disappeared. I found it impossible to determine the 

 exact number of hairs, but convinced myself that there were more 

 at the apex of the shorter antenna than at that of the longer^ 

 which in accordance with this theory ought to be the case. 



It is much to be wished that we had some definite system of 

 nomenclature for the appendages and parts of appendages of 



