216 PHOLADID.Z. 
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, 11 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 poimt to engage attention is the muscular struc- 
ture, which, with shght 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, bemg post-medial and 
fixed in the internal hollows of the auricles, showmg therein 
when removed well-marked cicatrices: this muscle throws off 
elastic ribands, which proceed on the lateral parts of the mantle 
to that pomt 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 pomt of support for the retractors of the compara- 
tively short siphons, and also the fulcrum for the pallets that 
are firmly fixed laterally therem, and undoubtedly serve to 
compress and relax the siphons. It is necessary to observe, 
that if the very long mantellar tube was not firmly attached, 
the points d’appui of the pallets and retractors would be lost, 
and the long, linear branchiz drawn together in the tube in 
such a mass as to impede the passage of the water and other 
functions. The posterior sphincter in Dentaliwm is analogous 
in its uses; and though the hemispherical valves of Teredo 
