104 ZOOLOGY. 



There are four antennae, of which the second pair is su2>posed 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 suj)posed genus Zoea^ to be only the young of the 

 common crab ; and Jurine had observed the change which the small fresh 

 water sjDecies undergo. The young of Lerntea 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 Malacostraca^ in which there are ten or fourteen 

 legs. 



Eiitomostraca . 



Order 1. Eotatoria (or Rotifera). 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 vibrillfe. 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 

 Rotifera {j)!. 75, fig. 16) includes species less than half a millimetre long, 

 which live in water, or among damp moss. The mouth and tail are cajtable 

 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 vibrillte. When at rest they afiix themselves by the 

 tail and bring their food within reach in the currents caused by the vibrillse. 

 Burmeister, in his work on the Organization of Trilohites., 1843, places the 

 Rotatoria as the lowest of the Crustacea, and next to them the CirrJiojwda^ 

 under the belief that the latter do not form a distinct class ; and in 1842, 

 J. E. Gray j^laced the Cirrhopoda here. These animals, and the Eotatoria, 

 are monoicous. 



Order 2. Cormostomata. Sexes separate, mouth with a suctorial beak, 

 carajjace generally in a single piece, feet adapted for holding, walking, or 

 swimming. Parasitic upon fishes, and undergoing metamor23hosis. The 

 order, as given here, includes two of Milne Edwards's orders, which contain 

 five families conjointly. It includes Baird's legion Pceciloj)Ocla^ 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. ii. 

 p. 53.) 



The Lerncddm {pi. 76, figs. SO, 81) are without eyes ; the head is small 



and the thorax large, the sej^aration being sometimes obscure, and the 



thoracic organs are rudimentary. The single pair of antennas is sometimes 



absent, and the number of foot-jaws is two or four. The body is curiously 



308 



