|408 CRUSTACEA. 



progress by locomotion. This then, is an example beyond dispute of 

 a system overgrown through the vegetative process, so as to be too 

 much for the motive energies within. The Lernseoids afford a similar 

 illustration of this principle. 



For the same reason, therefore, as in the Bopyri, the Medusa?, the 

 Lernseoids, and the Limuli, we cannot compare the actual mean size 

 of the adult Cirripeds with those of the other primary types. We 

 should rather infer the mean normal size for such a comparison, from 

 the size of the young before it becomes sedentary, or from that of free 

 males, if such exist. Such males are announced by Darwin, as actu- 

 ally occurring in some species. Moreover, they are very minute, 

 varying from a line to half a line or less in length. This, therefore, 

 is some reason for taking as the mean normal size, the same as given 

 for the Entomostraca. 



& fifth primary type includes the Rotatoria. In these animalcular 

 species, the mouth includes a pair of mandibles and often a rudimen- 

 tary pair of maxillse; and consequently, the cephalic portion may 

 contain the same number of annuli as in the Daphnia group, with 

 which group many of them have near relations. They have usually 

 an articulated abdomen, furcate at extremity, like the Cyclopoids. 

 The grand point of inferiority to the Entomostraca, evincing the more 

 infinitesimal character of the system of life within, is the absence of 

 all thoracic appendages or legs. The organs of locomotion are simply 

 cilise arranged about the head; and it is quite probable that two sets 

 (or more) of them correspond to the second pair of antennae, as these 

 are organs of prehension and motion in many Entomostraca. In Cal- 

 lidina, there are two sets, some distance from the extremity of the 

 head, which may have this relation; and the two sets in the true 

 Rotifers may also be of this character. In others, the corresponding 

 parts are actually somewhat elongated. 



The species vary in size from a line to a sixtieth of a line. Pro- 

 bably one-ninth of a line is the average size. 



The actual relation of the Rotatoria to the Entomostraca (which 

 view the author sustained in his Report on Zoophytes (1845)), can 

 hardly be doubted by those who have the requisite knowledge of the 

 lower Crustacea for comparison. The structure of the body, the 

 jointing and form of the abdomen, when it exists, the mandibles, and 

 alimentary system, the eyes when present, — all are Crustacean ; and 



