746 



ARTHROPODA 



PHYLUM VII 



Family 5. Verrucidae Darwin. 



Shells sessile and composed of six pieces. Of the scuta and terga only those of one 

 side are free the others being fused ivith the rostrum or carina. 



The solitary genus Verruca Schum., ranges from the Cretaceous to Recent. 



Family 6. Balanidae Darwin. (Acorn Barnacles). 



Shell ohtusehj conical, circular or oval in cross section, with broad, often calcareous 

 and cellular base; composed of foiir to ten ^'- compartments" more or less completely fused 



at their sides, and tivo pairs of free terga and scuta 

 which close the upper aperture like an operculum. 



Of the lateral plates which compose the crown- 

 shaped immovable test, two are designated as carina 



Pic. 1440. 



Diagram of the shell of Balanus. B, 

 Basis ; C, Carina ; CL, Carino - lateral 

 oompartinent ; L, Lateral compartment ; 

 It, Rostrum ; KL, Rostro-lateral com- 

 partment. Eacli valve or " compart- 

 ment " consists of a central ' ' paries " (p) 

 flanked by " alae " (ri) or "radii " (/■). 



... OG 



Fig. 1447. 



Scutini) and tergum of Bakmus. A, External aspect of tergum, 

 showing "spur" below and "beak" above. £', Internal view of 

 scutum, showing muscular scar {li). C, Internal view of tergum 

 (after Darwin). 



and rostrum, the pieces lying between and occurring in pairs being called lateralia. If 

 ^ J, additional plates are inserted 



among the lateralia, they are 

 termed according to their position 

 rostro- or carino - lateralia. The 

 scuta and terga lie free on the 

 Irnck of the animal, and in fossil 

 forms are generally lost. They 

 have a very characteristic form, 

 and hence are of great systematic 

 imiJortance. Since among fossil 

 species, however, only the mar- 

 ginal plates are for the most part 

 preserved, the determination of 

 their structural characters is often 

 quite uncertain. 



Balanus Lister (Figs. 1446- 

 1449). Shell low, conical or 



cylindrical, composed of six pieces. Opercular plates sub-triangular ; base membranous 



or calcareous. Eocene to Recent. 



Protobalanus Whitf. Affinities doubtful. Composed of twelve jslates, of which 



the carina is the largest ; rostrum small, lateralia in five pairs, fused only near the 



base. Middle Devonian. 



Acasta Leach. Sliell composed of six solid pieces. Base calcareous, cuii-.sha])ed ; 



epizoic on Sponges and Alcj^onarians. Pliocene and Recent. 



Fig. 1448. 



Balanus coneavus Broun. Crag; Sutton, England. .1, Shell. 

 iJ, Tergum. r, Scutum. Vi (after Darwin). 



