104 ZOOLOGY. 
There are four antenns, of which the second pair is supposed to be ~ 
connected with the sense of hearing, because there is near the base a cavity 
filled with a liquid, and receiving a special nerve. It has an exterior orifice 
closed with a membrane, which may be compared to a tympanum. 
The Crustacea are oviparous; and when the young leave the egg, some 
resemble the adult of the species to which they belong, although most of 
them undergo a metamorphosis. Mr. J. V. Thompson, of Cork, discovered 
an individual of Bosc’s supposed genus Zoea, to be only the young of the 
common crab; and Jurine had observed the change which the small fresh 
water species undergo. The young of Lernea and the allied genera are 
much like those of Cyclops. 
There are two sections of the Crustacea: Entomostraca, in which the — 
number of legs varies ; and J/alacostraca, in which there are ten or fourteen 
legs. 
Entomostraca. 
Orver 1. Roratoria (or otifera). This order has already been alluded 
to (pp. 9 and 25) in treating of the Infusoria. It is divided into such as are 
naked and such as have a shield, and each of these series has three sections 
according to the arrangement of the vibrille. According to Ehrenberg, 
this order contains fifty-five genera, divided into eight families ; but Dujardin 
reduces the families to five, and the genera to twenty-four. The genus 
fotifera (pl. 75, fig. 16) includes species less than half a millimetre long, 
which live in water, or among damp moss. The mouth and tail are capable 
of holding, so that the animal can move by attaching each end alternately, 
as in the leeches. They have also the power of swimming through the 
water by means of the vibrille. When at rest they affix themselves by the 
tail and bring their food within reach in the currents caused by the vibrille. 
Burmeister, in his work on the Organization of Trilobites, 1843, places the 
Lotatoria as the lowest of the Crustacea, and next to them the Currhopoda, 
under the belief that the latter do not form a distinct class; and in 1842, 
J. E. Gray placed the Cirrhopoda here. These animals, and the Rotatoria, 
are monoicous. . 
Orper 2. Cormostomata. Sexes separate, mouth with a suctorial beak, 
carapace generally in a single piece, feet adapted for holding, walking, or 
swimming. Parasitic upon fishes, and undergoing metamorphosis. The 
order, as given here, includes two of Milne Edwards’s orders, which contain 
five families conjointly. It includes Baird’s legion Pccilopoda, which is 
divided into two orders, five tribes, and eleven families, as given in his 
Natural History of British Entomostraca, 1850. This order has been 
named Siphonostoma, but as this designation has been otherwise employed, 
that of Professor J. D. Dana is adopted. This author gives it as a sub- 
order containing four tribes. (Proceed. Am. Acad. of Arts and Sci. vol. i. 
p: 53.) 
The Lernarde (pl. 76, jigs. 80, 81) are without eyes; the head is small 
and the thorax large, the separation being sometimes obscure, and the 
thoracic organs are rudimentary. The single pair of antenne is sometimes 
absent, and the number of foot-jaws is two or four. The body is curiously 
308 
