D. Dana on the Classification of Crustacea. 25 



lutely no greater than that of the male ; and consequently, the 

 capabilities of locomotion will be ten thousand times less, or the 

 female will move but a ten-thousandth of an inch at the most, 

 while the male is moving one inch, a fact with regard to them, as 

 any one is aware of who has seen the incapability of the female 

 to make any 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 Medusse, 

 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 seden- 

 tary, or from that of free males, if such exist. Such males are 

 announced by Darwin, as actually 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, 



A fifth primary type includes the Rotatoria. In these animal- 

 cular species, the mouth includes a pair of mandibles and often 

 a rudimentary pair of maxillae ; 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 Ento- 

 mostraca, 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 ciliae arranged about the 

 head ; and it is quite probable that two sets (or more) of them 

 correspond to the second pair of antennas, as these are organs of 

 prehension and motion in many Entomostraca. In Callidina, 

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

 which may have this relation ; and the two sets in the true Roti- 

 fers 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. 

 Probably one-sixth 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 struc- 

 ture of the bod}r, the jointing and form of the abdomen, when 

 it exists, the mandibles, and alimentary system, the eyes when 

 present, — all are Crustacean ; and a slight transformation of some 

 Entomostraca — an obliteration of the legs and substitution of 

 locomotive cilise — would almost turn them into Rotatoria. 



SECOND SERIES, VOL. XXII, NO. 64. — JULY, 1856. 4 



