216 



that part of the mantle which is external to the shell ; never- 

 theless they are protected by the tubular case, which,, as 

 I have stated, is an integral portion of the hard parts of 

 the animal, not merely protective or accessorial. We have 

 thus a complete equivalent for the bivalve shells of the 

 Pholades, in which the siphonal apparatus commences at 

 the posterior end of the shell, deriving their retractors from 

 offsprings of the medial adductor; whereas in Teredo the 

 retractors have their source from a particular muscular 

 sphincter at the posterior end of the tubular mantle in which 

 the pallets are inserted, and have nothing of the nature of 

 an adductor muscle, as the tube to which they are fixed is a 

 perfect cylinder. 



The next point to engage attention is the muscular struc- 



ture, which, with slight exceptions, scarcely differs from Pholas. 



The two principal masses of muscles are those of the foot and 



the adductor ; the latter is a powerful fibrous mass of bright 



red filaments, as Sir Everard Home states was the colour of 



the species he examined ; it embraces the hinder part of the 



mantle within the hemispherical valves, being post-medial and 



fixed in the internal hollows of the auricles, showing therein 



when removed well-marked cicatrices : this muscle throws off 



elastic ribands, which proceed on the lateral parts of the mantle 



to that point of the tubular mantle where the sphincteroidal 



muscle is fixed, and of which it is probably the origin : this 



last muscle is a most important one ; by being permanently 



fixed to the animal and the posterior end of the protective tube, 



by the oval-shaped fillets springing from the sphincter muscle, 



it is the point of support for the retractors of the compara- 



tively short siphons, and also the fulcrum for the pallets that 



are firmly fixed laterally therein, and undoubtedly serve to 



compress and relax the siphons. It is necessary to observe, 



that if the very long mautellar tube was not firmly attached, 



the points d'appui of the pallets and retractors would be lost, 



and the long, linear branchise drawn together in the tube in 



such a mass as to impede the passage of the Avater and other 



functions. The posterior sphincter in Dentalium is analogous 



in its uses ; and though the hemispherical valves of Teredo 



