CRUSTACEA. 493 



history of the four sub-orders, viz. Thoracica, Abdominalia, 

 Ap. -.la. and Rhizocephala. 



Thoracica. The just hatched larva; at once leave the egg 

 lamella: of their parent. They pass out through an opening in 

 the mantle near the mouth, and during this passage the shell of 

 the parent is opened and the movements of the cirriform feet 

 cease. 



The larval stages commence with a Nauplius 1 which, though 

 regarded by Claus as closely resembling the Copepod Nauplius 

 (figs. 231 and 232 A), certainly has very marked pecularities of 

 its own, and in some respects approaches the Phyllopod 

 Nauplius. It is in the youngest stage somewhat triangular in 

 form, and covered on the dorsal side by a very delicate and 

 hardly perceptible dorsal shield, which is prolonged laterally 

 into two very peculiar conical horns (fig. 231 ///), which are the 

 most characteristic structures of the Cirriped Nauplius. They 

 are connected with a glandular mass, the secretion from which 

 passes out at their apex. Anteriorly the dorsal shield has the 

 same extension as the body, but posteriorly it projects slightly. 



An unpaired eye is situated on the ventral surface of the 

 head, and immediately behind it there springs a more or less 

 considerable upper lip (Ib), which resembles the Phyllopod 

 labrum rather than that of the Copepoda. Both mouth and 

 anus are present, and the hind end of the body is slightly forked 

 in some forms, but ends in others, e.g. Lepas fascicularis, in an 

 elongated spine. The anterior of the three pairs of Nauplius 

 appendages (At*) is uniramous, and the two posterior (At* and 

 md) are biramous. From the protopodites of both the latter 

 spring strong hooks like those of the Copepod and Phyllopod 

 Nauplii. In some Nauplii, e.g. that of Balanus, the appendages 

 are at first not jointed, but in other Nauplii, e.g. that of Lepas 

 fascicularis, the jointing is well marked. In Lepas fascicularis 

 the earliest free Nauplius is enveloped in a larval skin, which is 

 thrown off after a few hours. The Nauplii of all the Thoracica 

 undergo a considerable number of moults before their appendages 

 increase in number or segmentation of the body appears. During 

 moults they grow larger, and the posterior part of the 



1 Alepas squalicula is stated by Koren and Daniclsscn io form an exception to 

 this rule, and to leave the egg with six pain of appendages. 



