280 



LECTUllE XXr. 



the valves at the depths at which they may be destined to live. The 

 Terehratula, in this res23ect the most remarkable of Bivalves, has an 

 internal skeleton superadded to its outward defence, by means of 

 which additional support is afforded to the shell, a stronger defence 

 to tlie viscera, and a firmer basis of attachment to the spiral arms. 



LECTURE XXI. 



LAMELLIBKANCHIATA. 



The relation of the contained soft parts to the bivalve shell of the 

 Brachiopoda is such that, in the Terehratula, the perforated valve 

 must be regarded as the inferior or ventral one, and the imperforate 

 or shorter valve the dorsal one. In the LameUihranchiata one valve 

 is apjolied to the right, and the other to the left, side of the animal. 

 In the common oyster and the Anomia, which are fixed and mo- 

 tionless, as in the Brachiopoda, the two lobes of the mantle are as 

 little united with each other, and there is as little evidence of any lo- 

 comotive organ or foot. The spiral brachia would seem to be reduced 

 to two shorter and more simple processes, and the inferior labial fold 

 to be produced on each side to the same length, so that there is a pair 

 of labial processes on each side the mouth. These appendages have 

 no internal calcareous support, which, by being bent, could open the 

 valves ; nor are they long enough, save in some species of Anomia, 

 to be protruded from the shell. In other Lamellibranchiate Bivalves 

 the labial processes are short and simple. In all the present class 

 the divarication of the valves is provided for by the insertion of an 

 elastic substance at their hinge ; and the valves are closed by the 

 contraction of short and thick subcircular muscles, thence called 

 the adductors. In the common oyster, and some other allied Bivalves, 

 there is but one adductor muscle. The visceral mass occupies about 

 half the cavity of the shell next the hinge. The rest of the interspace 

 of the pallial lobes being almost wholly occupied by the branchial 

 laminee, which are four in number, of a crescentic figure, placed two 

 on each side of the visceral mass. This is the characteristic condition 

 of the respiratory organs in the present class of Acephalous Mollusca, 

 and from which it derives its name. 



In the oyster the mouth is continued by a short oesophagus to an 

 expanded stomach, from which numerous ramified hepatic follicles 

 are developed. The intestine, after describing a few convolutions, is 



