640 CLASS X. 



Thoracica three families are included the Balanidce or sessile Cirri- 

 peds, the Verrucidce, (the genus Clysia LEACH, Creusia LAM., Ver- 

 ruca SCHUMACHER, SCHUM. Essai oTun nouveau Systeme des Habita- 

 tions des Vers testaces, 1817. Copenhav.), remarkable for their 

 quite asymmetrical shell, and the Lepadidce or pedunculated Cir- 

 ripeds. In the second order of Abdominalia the seventh or last 

 cephalic segment is quite distinct, and bears rudimentary organs 

 answering to the first pair of foot-jaws in ordinary crustaceans, 

 of which organs, and of the segments supporting them, there is 

 no trace in the Thoracica; the seven succeeding thoracic seg- 

 ments are destitute of any appendages ; but the three segments 

 of the abdomen bear three pairs of cirri. In the metamorphosis 

 all the first changes are merely indicated by changes of form 

 in the egg-like larva, without the development of distinct organs, 

 and the last or pupal condition, which is attained within the sac of 

 the parent, is very peculiar from the entire absence of rotatory 

 limbs. There is only one genus Cryptophialus DARWIN, and one 

 known species Cryp. minutus DARW., Balanidce, p. 566, Tab. 23, 

 24. The third order Apoda is the most peculiar of all, it resembles 

 the larva or maggot of a fly, the carapace is reduced to two separate 

 threads for attachment. The last cephalic, the seven thoracic, and 

 the three abdominal segments are all destitute of appendages. The 

 single known species, Proteolepas bivincta, is parasitic within the sac 

 of Alepas cornuta, DARWIN Lepadidce, p. 165, Tab. 3, fig. 6. See 

 DARWIN Balanidce, pp. 20 22.] 



ORDER V. -Cladocera LATE. 



Body compressed, included in a horny shell, divided into two 

 parts by a dorsal fold, without hinge. Head free, produced below, 

 terminating in a species of rostrum. Two small antennas at the 

 apex and two larger at the base of head, split into two or three 

 branches. Feet foliaceous, with four or five pairs. Abdomen ter- 

 minated posteriorly by two setaceous appendages. 



Family XI. Daphnidea STRAUS. The characters of the order 

 are those of the single family. Single compound eye. Larger 

 antennae ramose, serving like oars for swimming. Ova in females 

 situated on the back in a space between the shell and the segments 

 of the thorax. 



Polyphemus MUELL., Cephaloculus LAM. Eye single, large, 

 occupying almost the whole head. Larger antennas bifid. Feet 

 eight, exsert from shell. Tail reflected, bisetose at the apex. 



