ROCK-BORING ORGANISMS 



originalh^ enclosed in a small calcareous head" (Seymour Sewell, 

 1926). B)' this means the animal excavates a cavity into which the 

 contracted peduncle fits, with the opercular plates just flush with 

 the surface, so that even when common these barnacles are initially 

 most difficult to detect. The peduncle is attached to the boring by 

 way of a calcareous basal plate a little distance from the tip on the 

 carinal side (Fig. 1). This supplies the necessary purchase for bor- 



FiG. 1. LitJiotrya nicoharica, probable appearance when withdrawn within 

 boring. (Modified after Seymour Sewell, 1926.) 



ing, although, of course, the position of attachment must continually 

 alter during growth (the same problem arises in the case of the 

 boring bivalve Tridacna crocea, as described below). The process 

 of boring must be purely mechanical. Like other barnacles, LitJio- 

 trya feeds on zooplankton organisms by means of its setous thoracic 

 appendages. 



Bivalve molluscs. The basic structure of these animals ideally 

 fits them for boring. Enclosure of the entire body by the mantle and 

 shell has led to loss of the head and enlargement of the gills, which 

 have assumed the added function of collecting food consisting of 

 phytoplankton. Except in a few primitive and some specialized 



