THE SESSILE BARNACLES. 



113 



teeth on the anterior margin (fig. 286). The later cirri have four 

 or at most five pah's of spines (fig. 28e). The second joint of the 

 pedicel of cirrus vi has an upwardly directed tooth near the proximal 

 posterior part, usually with a few minute ones above it (fig. 28c.) 

 B. trigonus may usually be readily recognized by the triangular 

 orifice, the ribbed and colored walls, the rows of little pits in the scutum 

 (which, however, may rarely be wanting, or in very old specimens 

 they may be obscured or removed by erosion), and the thin, flat 

 tergum with a broad spur. The shape of the orifice varies, two angles 



FIG. 28. BALANUS TRIGONUS. a, SEVENTH SEGMENT OF ANTERIOR KAMUS, CIERUS in. &, 



EIGHTH SEGMENT OF CIRRUS IV. C, SECOND SEGMENT OF THE PEDICEL OF CIRRUS VI, "WITH 

 PART OF THE LOWER SEGMENT AND BASES OF THE KAMI, d, SEVENTH SEGMENT OF AN- 

 TERIOR RAMUS, CIRRUS HI, CAPE ST. LUCAS. , A MEDIAN SEGMENT OF CIRRUS VI. ALL 

 FIGURES EXCEPT d FROM WEST INDIAN SPECIMEN. THE RIGHT SIDE IS ANTERIOR. 



of the triangle often being cut off, forming two short facets, changing 

 the triangle to a pentagon with two very long sides. 



It is very frequently associated with other barnacles, and is one 

 of the forms often found on ships' bottoms. Sometimes it forms 

 masses on shells, which may be completely covered excepting the 

 aperture. Doctor Kriiger, who has given an excellent account, with 

 figures, found it on crabs from Japn, where it is a common species. 

 He also records a group of small individuals embedded Acasta-like 

 in a sponge. He independently reached Fritz Miiller's idea that the 

 spines on the third cirrus were evolved to break away sponge-spicules 

 from the orifice. 



