BRACHIOPODA. 279 



testes in the male have the same form and disposition : they differ 

 from the ovaria in their closer texture and their whiter colour. 



On comparing together the three existing genera of Brachiojioda 

 which have been selected for anatomical demonstration in the present 

 Lecture, we may perceive that the modifications which are traceable 

 in their respective organisations bear relation to the different situ- 

 ations which they occupy in the sea. 



The Liiuiula, living usually near the surface, and sometimes where 

 it would be left exposed by the retreating tide, if it were not buried in 

 the sand of the shore, must meet with a greater variety and abundance 

 of animal nutriment than can be found in the deeper waters, where 

 the Terebratula usually resides. Hence its powers of prehension are 

 greater ; and Cuvier suspects that it may even enjoy a species of 

 locomotion from the superior length of its peduncle. The organisation 

 of its mouth and stomach indicates the molecular character of its 

 food ; but its convoluted intestine shows a capacity for extracting a 

 quantity of nutriment proportioned to its superior activity and to the 

 greater extent of its soft parts. The more obvious and comj^lex 

 respiratory apparatus is in exact harmony with the above conditions 

 of structure and habits. 



With regard to the Orbicula, and more especially the deep sea 

 species of Terebratula, both the respiration and nutrition of such 

 animals which exist beneath a pressure of from sixty to ninety fathoms 

 of sea water, are subjects suggestive of interesting reflections, and lead 

 one to contemplate with less surprise the great strength and com- 

 plexity of some of the minutest parts of the frame of these diminutive 

 creatures. In the unbroken stillness which must pervade those 

 abysses, their existence must depend upon their power of exciting a 

 perpetual current around them, in oi'der to dissipate the water already 

 laden with their effete particles, and to bring within the reach of 

 their prehensile organs the animalcules adapted for their sustenance. 

 The actions of the Terebratula and Orbicula, from their attachment 

 to foreign bodies, are confined to the movements of their brachial 

 and branchial filaments, and to a slight divarication and sliding nio- 

 tion of their protecting valves ; the simplicity of their digestive ap- 

 paratus, the still greater simplicity of their branchiiP, 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, how- 

 ever, remarkable for the strong and unyielding manner in which they 

 are connected together. The muscular system is much more complex 

 than in ordinary bivalves, and is remarkable for the compactness of 

 its fibre and the density of the glistening tendons. Here is obviously 

 an apparatus of sufficient power to effect the requisite motions of 



T 4 



