J. D. Dana on the Classification of Crustacea. 13 
lutely no greater than that of the male; and consequently, the 
«apabilities of locomotion will be ten thousand times less, or the 
female wil move buta 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 incapabihty 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 Lernzoids afford a similar illustration of this principle. 
For the same reason, therefore, as in the Bopyri, the Medusse, . 
the Lernzoids, 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 1t 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 inlength.  'lhis, therefore, 1s some reason for taking 
as the mean normal size, the same as given for the Entomostraea, 
À. fifth primary type includes the RorATORIA. In these animal- 
cular species, the mouth includes a pair of mandibles and often 
à rudimentary pair of maxille; and consequently, the cephalie 
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, 1s the absence of all thoracie appendages or legs. 
The organs of locomotion are simply cili arranged about the . 
head; and itis quite probable that two sets (or more) of them 
correspond to the second pair of antennsze, 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 aine to à sixtieth of a line. 
Probably one-sixth of a line 1s the average size. 
"The aetual 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 body, 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 
Entomostraea—an obliteration of the legs and substitution of 
locomotive cilize—would almost turn them into Rotatoria. 
SECOND SERIES, VOL. XXII, NO. 64.—JULY,, 1856, 3 
