1022 



CRUSTACEA. 



Milne Edwards refers the four posterior pairs of natatory legs in 

 Nebalia to the abdomen ; but their close similarity to the thoracic 

 natatories in Cyclops, and also the resemblance between the following 

 part of the body in the two groups, satisfies us that these natatories 

 in Nebalia are properly thoracic, and that consequently the abdomen 

 of Nebalia is not abnormal in the number of its segments.* The 

 structure in certain Mysidse is strongly brought to mind. 



Chirocephalus and Artemia constitute one family, the Artemiadj;, 

 among the Artemioidea, and the Nebalia another, the Nebalidajs. 



The Apodoidea have sessile eyes, and although having a little of the 

 Nebalia and Cyclops habit, yet they are quite remote in their charac- 

 ters, and more resemble the Limuli in structure, especially in the folia- 

 ceous appendages below, the appearance of the shell, and the position 

 of the eyes, though not like them in the mouth. The abdomen is 

 straight, as in Cyclops, but its extremity is unlike any species of that 

 group. 



The Limnadioidea have a close resemblance in form to Cypris, and 

 the extremity of the abdomen is like that in Daphnia. 



The Phyllopoda should properly be arranged after the Lophyro- 

 poda. The great number of joints and appendages, as shown by 

 Agassiz, is a mark of inferior grade. It is a characteristic of larval 

 forms among insects, as distinguished from the perfect animal. It is 

 a peculiarity of the earlier Crustacean forms of the globe, when vege- 

 tative growth in animals often predominated over concentration. 



The three tribes of Lophyropoda appear to rank in the order above 

 given. The Cyclopoidea approach the nearest in general form and 

 structure to the Mysidse, and they may be considered their represen- 

 tatives among the Entomostraca. The Cyproidea are quite remote 

 in structure from any of the higher Crustacea. Moreover, they 

 resemble closely the immature forms of the Anatifidae, also a low grade 

 of Crustacea, a relation which indicates their own inferiority. The 

 Daphnioidea are evidently intermediate between the Cyclopoidea and 

 Cyproidea; for they have the large shell and habit of the latter, 

 with the natatory legs and exsert head of the former. 



* See our remarks on the homologies of Nebalia, page 41. 



