. Bulletin. Natural history; Natuurlijke historie. 8. BURROWS Unlike most endolithic bivalves, gastrochaenids secrete aragonitic burrow linings that form distinctive internal burrow shapes and commonly show considerable detail in sculpture and ornamentation. Typical internal bur- row shapes and burrow lining distributions are shown in Figure 33. The present observations largely confirm and extend those of Robertson (1963) for the three common Florida Keys gastrochaenids. Burrows of S. rostrata and G. (R.) ovata are less than twice as long as the shell and commonly curve gently in the dorsal di
. Bulletin. Natural history; Natuurlijke historie. 8. BURROWS Unlike most endolithic bivalves, gastrochaenids secrete aragonitic burrow linings that form distinctive internal burrow shapes and commonly show considerable detail in sculpture and ornamentation. Typical internal bur- row shapes and burrow lining distributions are shown in Figure 33. The present observations largely confirm and extend those of Robertson (1963) for the three common Florida Keys gastrochaenids. Burrows of S. rostrata and G. (R.) ovata are less than twice as long as the shell and commonly curve gently in the dorsal direction. In contrast, burrows of G. (G.) hians are several times longer than the shell and may be straight or sinuously curved. All three burrows show two well-defined parts, a pos- terior siphonal burrow and an anterior shell chamber. Aragonitic linings in the form of an annular diaphragm or pointed "baffles" may constrict the siphonal burrow at its junction with the shell chamber. As a result, shell mobility is restricted to the shell chamber. The shell chamber is only slightly larger than the shells in every dimension, and it varies from circular to slightly elliptical in cross-section (Fig. 34). This part of the burrow is wide enough to allow rotations of the shells about an anteropos- terior axis, and rotations by S. rostrata in partially opened burrows have been observed up to 90 degrees in either direction relative to the stationary foot. The lining of the siphonal burrow is smooth in G. (R.) ovata, but this. 1 cm. Fig. 33. Burrow cross-sections for adults ofSpengleria rostrata (a),Gastrochaena (Rocellaria) ovata (b) and Gastrochaena (Gastrochaena) hians (c). The coral substratum is indicated by stipling; the aragonitic burrow linings are indicated by solid lines. The absence of a burrow lining is shown by a dashed line. The solid lines drawn between the siphon tubes of Spengleria rostrata suggest the configuration of accretion banding in the aragonitic deposit
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