THE SESSILE BARNACLES. 7 



that in all cases ii'liere tlie rostrum has radii, or overlaps the adjacent corn- 

 jyartnicnts, it is a composite plate, forined by the concrescence of both 

 rostral hxtera Avith the true rostrum. This is the case with all recent 

 Balani(he and part of the Chtliajnalidse. It is inexact to term the 

 compartment formed by fusion of rostrum with the rostral latera a 

 "rostrum," in Balanus, for example, as it is not eciuivalcnt to the 

 rostrum in the less niodified (l>thamalida3, tlie Verruci(hi^, or the 

 pedimculate cirripedes. So long, hoAvever, as the true constitution 

 of the plate is understood, the inconsistent terminology is perhaps of 

 no great moment. 



The chief modifications in the arrangement of the compartments 

 due to concrescence or to elimination are represented diagrammatically 

 in figure 5. Having examined a very great majority of the known 

 species of sessile cirripedes, I find that the evidence supports Darwin's 





Fig. 5.— Diagrams showing the modes of imbkication and the homologies of compartments in 

 chthamalidie and b.vlanid^. c, carina, cl, carinol ateral compartment. i, lateral. t, 



rostrum, rl, ROSTROLATERAL. THE SECOND DIAGRAM REPRESENTS OCTOMERIS. 



views of tlie homologies of the compartments, so far as he definitely 

 expressed himself, and is opposcnl to the interpretations of Gnn^el.^ 



The question of homologies of the wall plates is by no means aca- 

 demic. Our conceptions of the classification and phylogeny of the 

 group depend upon our understanding of these homologies. 



The cirri are always well developed in sessile barnacles, and there 

 is much less diversity in the arrangement of the spines than in the 

 Lepadomorpha. The first two or three pairs of cirri bear dense, 

 brusliliko tufts of spines; the later cirri bear spines in pairs, two to 

 eight or ten pairs on each segment. The number of pairs of spines 

 is a useful specific character, though subject to some variation 

 in polymorphic species. In descriptions the comit is made in the 

 median third of the cirrus, where the maximum immbcr is found. 



I Gravel, Monographie des CirrhipMes, p. 194, fig. 213, where Professor Gruvel's views of the homologies 

 pf the compartments are given. 



