396 Dr. F. Miiller on Balanus armatus. 



which are seated close together upon the tube of a Serpula 

 [Eupomatus jiorihandas ^ F. MiilL), which bears, close to them, 

 two shells of B. imjyrovisus^ var. assimilis. This, which is the 

 commonest of all the Balanidie here, sometimes even occm*s in 

 Reniera as the companion of B, armatus. 



General appearance. — The shell oi Balanus armatus (fig.l), 

 as regards form and colour, is very variable in its general 

 appearance. For the most part, in this, as in other species, 

 this ditference is caused by the support upon which the animal 

 has domiciled itself. Hence the most regular shells are gene- 

 rally those imbedded in soft sponges, the development of 

 which never meets with any obstacles. They are usually 

 found here of a steeply conical form sometimes nearly cylin- 

 drical, the longitudinal diameter (from the rostrum to the 

 carina) generally rather greater than the transverse, the ros- 

 trum and carina nearly of equal height, the base always con- 

 cave, and generally in a high degree. In this respect, there- 

 fore, B. armatus agrees with the allied B. sponcjicola^ and 

 differs, like the latter, from the sponge-haunting Acastce, in 

 which the base is strongly convex. But even in Reniera very 

 divergent forms are not wanting. I have seen shells in which 

 the rostrum was only half as high, and others in which it was 

 twice as high, as the carina. 



The form of the shells seated upon the Carijoa is particu- 

 larly variable. It differs according as they are attached longi- 

 tudinally, or transversely, or obliquely to the stem of the polype, 

 which usually forms a deep furrow in their base. This is 

 elongated in the direction of the furrow, which, again, has an 

 influence upon the whole shell, so that even in its middle the 

 breadth varies from two-thirds to four-thirds of the length. 

 Not unfrequently the separate pieces of the shell are of very 

 different heights, all the pieces of one side being sometimes 

 twice as high as those of the other. Rarely the furroAV of the 

 base is closed, so as to form a complete tube. On one occasion 

 I found Balanus armatus adhering to the apex of a branchlet ; 

 and in this case the base formed a conical tube round the 

 branchlet, longer than half the height of the shell ; the dia- 

 meter of the base was only half the length of the orifice. In 

 other cases, again, the base is more than twice as long as the 

 orifice. Shells inflated in the middle also occur. A particu- 

 larly remarkable form was seated transversely upon the stem 

 of a Carijoa. The rostrum and carina are unusually broad, 

 almost equilaterally triangular ; they embrace the stem and 

 meet together beneath it upon one side in a sharp edge. The 

 walls of the lateral and carino-lateral pieces, on the contrary, 

 are quite narrow strips. But it would be necessary to figure 



