490 LECTURE XX. 



gula has a stratum of muscular fibres. For the modifications of the 

 muscular system in Orbicula, I may refer to my monograph on its 

 anatomy.* The four adductor muscles are distinct at both ends in 

 that genus and in Crania. 



In Terebratula the fringed portions of the spiral arms are not 

 immediately supported by the loop ; this serves only for the attach- 

 ment of the thin but firm aponeurotic membrane which forms the 

 true basis of support of the beautiful and peculiar organs in ques- 

 tion. 



The aponeurotic fold supporting the basis of the arms is perforated 

 by the cesophagus, the mouth opening downwards into the pallial 

 cavity below the beginning of the spiral fold : the alimentary canal 

 bends backwards over the basal fold, after perforating it ; the fold 

 thus intervening between -the oesophagus and intestine. The fila- 

 ments given off from the beginning of the arms, supported by the 

 basal fold are shorter than the rest. The aponeurosis, reflected from 

 the lateral to the spiral folds, forms the fore-part of the small visceral 

 cavity ; the pallial membrane is continued from the union of the 

 lateral with the basal folds, is stretched over the visceral inter- 

 space between the advancing crura, and is then continued backwards 

 towards the hinge, protecting the hearts and sinuses behind the trans- 

 verse fold. The lateral folds decrease in breadth as they advance 

 forwards : the spiral fold, which, at its beginning, is broader than the 

 broadest part of the lateral folds, gradually becomes narrower as it 

 approaches the termination of the spire. The lamellge of the several 

 above-defined parts of the aponeurotic supporting and connecting 

 brachial folds, separate when they reach the calcareous plate, and the 

 hollow muscular stems of the fringed arms closely surround and 

 adhere to these stems, being continued thence upon the border formed 

 by the roots of the fringe-filaments, where the aponeurotic character 

 is exchanged for that of a delicate membrane, which is finally lost 

 upon the filaments themselves. 



The fibres of the muscular walls of the brachial canal {fig- 186, m) 

 are arranged in a decussating double spiral, evidently adapted for 

 compressing the contained fluid, and thereby reacting upon the arm 

 of which the muscular canal forms the base. In the Terebratulidce, 

 like Rhijncho7iella psittacea, with free multispiral brachia, the fluid of 

 the canal being acted upon by the spirally-disposed muscles compos- 

 ing its parietes, is forcibly injected towai'ds the extremity of the arm, 

 which is thus unfolded and protruded outwards. In the species 

 resembling Ter. flavescens, the spiral portion may also be, in like 



♦ CCCr. vol. i. p. 149. 



