10 UNITED STATES NATIONAL MUSEUM BULLETIN 296 



round slits (fig. 33i), while the Trypetesidae have long slits usually- 

 bent at the finely pointed end (fig. 34c). 



The interior of the burrow generally extends nearly straight down 

 into the shelly material, the exception being the members of the 

 Family Trypetesidae; these are large species in which lateral flattening 

 has taken place. All species are able to extend their cirri from the 

 burrow aperture, but the trypetesids tend to make most of their 

 cirral strokes within the mantle cavity. 



The method of forming the burrow is largely by abrasion. This is 

 accomplished by chitinous teeth extending outward from the surface 

 of the mantle more or less distributed over the entire surface, but 

 particularly concentrated in some areas in some species. That abrasion 

 accounts for most of the burrowing is attested by the piles of shell 

 powder lying along the aperture on the surface of the host shell in 

 undisturbed specimens. The mantle teeth are renewed with each 

 molt, providing a fresh set to abrade the calcareous substratum. 



The fact that when the larva initially settles it does not bear 

 mantle teeth, together with the lack of production of powdered shell 

 material during initial burrowing, suggests that the initial penetration 

 into the substratum is accomplished by chemical dissolution (cf. 

 Berndt, 1903a, p. 405). Furthermore, the cemented attachment area, 

 needed for a good purchase on the shell to provide a base for moving 

 the mantle and needed for abrasion to be successful, is missing from 

 these first larval penetration stages. The first antennae, with which 

 the cyprid attaches to the shell, are withered and exposed externally, 

 and could not support abrasive movement. No acid or other solvent 

 has been isolated or identified, however. 



A pending issue of the American Zoologist is expected to contain 

 the papers presented at a symposium on the penetration of calcareous 

 substrates by invertebrates. Of direct interest will be the papers of 

 Seilacher, and Tomlinson. A recent paper by Turquier (1968) de- 

 scribes the use of carbonic anhydrase in burrowing by Trypetesa 

 nassarioides. 



Burrows normally are made in dead shelly material. In mollusks 

 the exterior of the shell is available, especially if the periostracum 

 has been eroded off. The interior of dead shells is also available, 

 although silting and burying of the shell is a problem to the barnacle. 

 This silting problem is solved sometimes by the services of a hermit 

 crab (especially in Trypetesa) which carries the shell around, circulates 

 sea water, and eats a variety of food close to the barnacle. Echinoids 

 are presumed to be dead when burrowed. Corals usually are attacked 

 at the dead bases and wherever the skeleton is exposed, although 

 Berndtia purpurea has been described in living coral (Utinomi, 1950a). 



