CIRRIPEDIA. 209 



In the Halocypridae also metamorphosis has almost disappeared, 

 the young when hatched being apparently already provided with all 

 the limbs, and only being distinguished from the adult by the 

 immature condition of the sexual organs, the small number of f ureal 

 hooks, and the slight development of the sexual characters (Claus). 



5. Cirripedia. 



The Cirripedia, which have been modified in a remarkable manner 

 in connection with their attached mode of life, have often been 

 placed in close relationship with the Copepoda, in accordance with 

 the views of Claus (No. 8). We must, however, in consequence of 

 the presence of the so-called Cypris stage (with a bivalve shell) which 

 occurs in their metamorphosis and brings about the transition from 

 the free to the attached life, assume for them a similarly-shaped 

 ancestral form, which we must then seek among the Phyllopoda. 

 Since the correspondence in structure and development between the 

 Cirripedia and the Copepoda may also be explained as the result of 

 adaptation in a similar direction (convergence), and as, on the other 

 hand, the shell in the Copepoda has undergone great reduction, 

 these two orders of Crustacea appear to us to be independent 

 derivatives of the primitive Phyllopodan stock. 



The metamorphosis of the Cirripedia begins with a true Nauplius stage, 

 characterised by the possession of the so-called frontal horns* and of a usually 

 triangular dorsal shield. The series of Nauplius stages proceeding one from the 

 other through several moults, ends in a Metanauplius in which can be recognised 

 the rudiments of the paired eyes, a fourth pair of limbs, and the subsequent six 

 pairs of thoracic limbs. Another moult and the larva appears in the free- 

 swimming Cypris stage which, after attachment, prepares for the transition into 

 the adult. This latter proceeds, after a further moult, from the attached Cypris 

 stage {pupa). 



A. Thoracica. 



The series of Nauplii belonging to the Balanidae (Fig. 102 A) are, 

 as a rule, simpler in shape than the much larger spined Nauplius of 

 Lepas, so that we shall commence with a description of the former. 

 The body shows no trace of external segmentation, and is covered 

 dorsally by a flat, triangular or somewhat oval shell, which, at 



* We must here point out that the possession of frontal horns is not a 

 universal distinctive feature of the Cirripedian Nauplius. Such horns are, 

 for instance, wanting in the Nauplius of Laura Gerardiae (p. 229), and in a 

 Rhizocephalan, found by Sluier, parasitic on an Ascidian {Sphaerothylacus 

 polycarpac). They are also wanting in that remarkable pelagic larva, of whose 

 systematic position we are still doubtful, which was obtained at Mindanao and 

 at first thought by Willemoes-Suhm to be the larva of Limulus, but was later 

 considered to be a Cirripede larva (No. 63). 



