268 RESPIRATION. 



the reach of their prehensile organs the animalciihi adapted 

 for their support. The actions of Terebratula and Orbicula, 

 from the firm attachment of their shells to foreign sub- 

 stances, are thus confined to the movements of their brachial 

 and branchial filaments, and to a slight divarication or slid- 

 ing motion of their protecting valves ; and the simplicity of 

 their digestive apparatus, the corresponding simplicity of 

 their branchiae, and the diminished proportion of their soft 

 to their hard parts, are in harmony with such limited powers. 

 The soft parts in both genera are, however, remarkable for 

 the strong and unyielding manner in which they are con- 

 nected together : the muscular parts are in great proportion, 

 and of singular complexity as compared with ordinary bi- 

 valves ; and the tendinous and aponeurotic parts are remark- 

 able for the similarity of their texture and appearance to 

 those of the highest classes. By means of all this strength 

 they are enabled to perform the requisite motions of the 

 valves at the depths in which they are met with. Terebra- 

 tula, which is more remarkable for its habitat, has an inter- 

 nal skeleton superadded to its outward defence, by means of 

 which additional support is aflTorded to the shell, a stronger 

 defence to the viscera, and a more fixed point of attachment 

 to the brachial cirri." * This interesting passage cannot 

 fail to please you, and its scope is, generally speaking, cor- 

 rect and just ; but if thence you infer that the Terebratulae 

 reside only in the unfathomed depths of the ocean, you will 

 be misled, for specimens of the recent species have been 

 taken from u^nder rocks that were left uncovered by a spring- 

 tide. Many bivalves too, which differ in nothing material 

 from littoral species, have their abode in very deep water ; 

 and Mr. Broderip furnishes me Avith a striking instance of 

 the various depths which species even of the same genus 

 affect. In describing some species of Clavagella, he says : 

 " Clav. austrahs was so near the surface at low water, that it 

 was detected by its ejection of the fluid ; Clav. elongata, 

 from the nature of the coral in which it is chambered, could 

 not have been living far beneath the surface ; whereas Clav. 

 lata was dredged up from a depth of sixty-six feet. Any 

 inferences, therefore, as to the state of submersion of a rock 

 during the life of the fossil species of Clavagellas which 

 there occur, should be made with caution by the geolo- 

 gist." f 



* Zool. Trans, i. 158. t lb. i. 2G7. 



