CLASSIFICATION OF CRUSTACEA. 9 



pods and Isopods, it leaves a large group, ranging from well-organized 

 forms to the sluggish worm-like Lernaea on one side, and to the 

 microscopic Rotifer on the other, the line of gradation to the former 

 being through the Caligacea, and to the latter through the Cyclops 

 and Daphnia groups. Moreover, while some species have mouths 

 with regular jaws, like the higher Crustacea, others have a trunk for 

 suction ; and in still others (Limuli) the jaws are but the basal joints 

 of the legs. 



May we, in view of these differences, retain all the species in a 

 common group, subdivided in accordance with the varieties of struc- 

 ture ? or shall we, as done by Milne Edwards, give the very highest 

 rank in classification to the character of the mouth, and so make our 

 first three grand divisions of Crustacea paramount to all others, the 

 following, 1, those with proper jaws (Crustace"s Maxille's) ; 2, sucking 

 species (Crustace's S^eurs) ; 3, those with the basal joints of the legs 

 acting as feet ? 



The differences here alluded to, are, in our estimation, of compara- 

 tively small importance. They are confined to the mouth, and are 

 simply an adaptation of the same organs to somewhat different modes 

 of life. The sucking Crustacea have the mandibles of other species, 

 although more slender and placed in an elongate trunk ; and all 

 other important characters are identical with those of certain maxil- 

 lated species of like form and grade of structure. 



In all Crustacea, the mandible is but a process from the basal 

 joint of a leg, and the maxillae are of like character. In the 

 higher species, the leg or jointed portion of the organ is short; 

 but among the lower, it often has a large development, and all the 

 maxillae may be like feet in form, and actually so in part of their 

 functions. It is, therefore, but a single step, a shade beyond, which 

 brings us to the Limuli, in which all the mouth organs are feet, and 

 similar in form, the basal joints of which act together as jaws. In 

 the Calani and Cypridinae, true mandibulated species, the maxillae are 

 much enlarged, and the mandibles have long, jointed, fooWike appen- 

 dages. Facts of this kind are too well known to require repetition, 

 and they need but be appreciated, we think, to make the impropriety 

 evident of laying that stress upon this characteristic which is done in 

 the classification just alluded to. 



We acknowledge that if this adaptation to suction occurred among 

 the superior grades of Crustacea, it should have a high value in classi- 



3 



