THORACICA. 215 



shifts to the base of the eye-stalk formed from part of the head of 

 the larva, and here takes the form either of a closely-packed coiled 

 gland (Lepas pectinata), or else of widely scattered cells connected 

 only by the efferent duct. On account of its structure, the cement- 

 gland must be regarded as an accumulation of unicellular glands on 

 a much-branched common duct, the single glandular cells being sessile 

 on this duct, like grapes. 



We are still very much in the dark as to the genesis of this gland and its 

 morphological derivation. Attempts have been made to derive it from one of 

 the two typical pairs of Crustacean glands (antennal and shell-glands), although 

 such a homology is attended by considerable difficulty on account of the aperture 

 of the gland occurring on another limb. Clatjs (No. 8) finds, within the shell- 

 fold of the free-swimming Cypris stage of Lepas australis, a coiled gland-like 

 cell strand, and is disposed to homologise this with the shell-gland of other 

 Entomostraca, while he conjectures that the cement-gland which is recognisable 

 in later stages is derived from this cell-strand. Willemoes-Suhm, on the 

 contrary (No. 62), found, even in the Nauplius stage, a paired glandular mass 

 lying at the sides of the upper lip, out of which, according to him, the cement- 

 gland develops. While authorities thus hold conflicting views, we shall do well 

 for the present to regard the cement-gland as a peculiar structure found in the 

 Cirripedia, and to abstain from attempting to homologise it with the glands 

 of other Crustacea. 



A great change occurs in the structure of the limbs which 

 surround the oral aperture of the Nauplius. The second antennae 

 seem altogether to disappear (if they are not to a certain extent, as 

 Pagenstecher (No. 58) conjectured, retained in the palp-like 

 appendages of the upper lip). The actual mouth-parts, together 

 with the upper lip, are already shifted on to a slightly projecting 

 oral cone, and appear in the form of three pairs of truncated rudi- 

 ments, in which we recognise the future mandibles, the first maxillae, 

 and the lower lip which results from the fusion of the second 

 maxillae. In what way these mouth -parts are derived from the 

 limbs of the Metahauplius stage is still far from clear. The most 

 probable view appears to us to be that of Claus (No. 8), according to 

 which the mandibles are derived from the basal joints of the third 

 pair of Nauplius limbs, while the outer segment of the rudimentary 

 limbs following these in the Metanauplius (Fig. 102 B, mx) yield the 

 first maxillae. The under lip, on the contrary, is said to rise from a 

 rudiment on the inner side of this limb. We should thus have to 

 assume that, in this imperfectly developed limb, we have the rudi- 

 ments of the first and second pairs of maxillae crowded together. 

 Metschnikoff (No. 53) also derives two pairs of limbs from this 

 rudiment, but identifies them with the mandible and maxilla of 



