maxillipeds always form simple stems furnished with procurved setse or claws, and 

 on them may be easily distinguished a basal and a terminal part, the former 

 generally biarticulate, the latter multiarticulate. As a rule, the posterior max- 

 illipeds are more slender than the anterior; but in some cases this pair is much 

 reduced in size, whereas the anterior pair may be very powerfully developed. 

 Of legs generally 5 pairs occur, the 4 anterior of which are always natatory 

 and biramous, whereas the last pair are only exceptionally of that character, 

 but in the female are more generally much reduced, not infrequently even wholly 

 absent, and in the male are transformed into prehensile organs, by the aid of 

 which the spermatophores are grasped and transferred to the female. The normal 

 number of caudal setre is 5 on each ramus ; but this number is sometimes 

 reduced to 4 or even to 3. In every case, in addition to the true caudal setse, 

 a slender bristle may be observed on the inner corner of each ramus, gener- 

 ally extending dorsally, more rarely ventrally. The ovisac, when present, is 

 always simple, more or less flattened in form, and attached to the ventral face 

 of the genital segment. 



As to the internal organisation, the Calanoida are prominently distinguished 

 by the presence of a well-developed heart occurring dorsally at the limit between 

 the 1st and 2nd pedigerous segments. The male generative organ is asymmetrical, 

 the efferent duct being located on one side, generally the left. The asymme- 

 trical structure of the last pair of prehensile legs is apparently in accordance 

 with this circumstance. 



By far the greater number of the Calanoida are marine ; but there are also 

 several forms peculiar to fresh or brackish water, especially of the families 

 Centropagidce, Diaptomidce and Temoridcv. In the Oceans some forms, especially 

 of the genus Galanus, often occur in immense shoals, forming the bulk of the 

 "Zooplankton". These animals therefore are of great importance as fish-food, and 

 in some cases even form the chief nourishment of the great whales. 



The exact subdivision of the Calonoida is connected with no small 

 difficulty, owing to the great variation in the leading characters. Dr. Gies- 

 brecht, in his beautiful work, has however tried to make such a subdivision. His 

 two primary sections, Amphascandria and Heterarthrandria, are very natural, 

 though I think that a 3rd such section ought to be added, comprising those forms 

 in which the anterior antenna? are alike in both sexes. For this section I 

 propose to use the name Isokerandria, applied by Dr. Giesbrecht also to a 

 section of the Cyclopoida. According to Dr. Giesbrecht's classification the 1st 

 section only includes a single family, the Calanidce, whereas the 2nd com- 

 prises 3 families, viz., the Centropagidce, C&ndacidce and Pontellidce, The 2 first 



