194 THE ACEPHALA. $ 178. 
As the points of the insertion of these muscles do not always correspond 
with the two valves,” their direction is often oblique, or, they sometimes 
even cross each other.© Orbicula and Lingula, which want both the liga- 
ment and hinge, have this disposition of the adductors which terminate 
usually by a delicate tendon, and the contraction of either of these muscles 
alone, produces the lateral movements of their valves. 
The spirally-pointed tentacles of the Brachiopoda are moved by a par- 
ticular apparatus. The fringes of these organs are inserted upon a carti- 
laginous tubular prolongation which tapers to a point. . This is closed at 
both extremities and contains a liquid, which, by the contractions of the 
circular muscular fibres, is propelled from the base to the extremity, thereby 
unrolling the spiral turns.” These tentacles here certainly take the place 
of the elastic ligament of the bivalves, for their extension probably tends 
to slightly open the valves. ; 
With the Lamellibranchia, the tentacles which are not rolled, are ar- 
ranged quite differently. Like their branchiae, they are scarcely at all 
irritable, — this being due to the fewness of their muscular fibres. But in 
the mantle these fibres are very abundant, and especially near their free 
borders. This is true also of the Siphon, in which both longitudinal and 
circular fibres can be easily seen; here, two very distinct flattened muscles 
arise from the base of the siphon and are inserted upon the two valves, ex- 
ternal to the posterior adductor muscle: these serve as a Retractor siphonis. 
§ 178. 
Very many of the Lamellibranchia have a highly-developed organ of 
‘locomotion, —the foot. This isa muscular prolongation from the ventral 
surface, which passes obliquely forward to be inserted upon the internal sur- 
face of the back of the shell, by four, rarely more, tendinous cords.” 
These cords surround the abdominal viscera, and becoming gradually 
thicker and more muscular, finally blend with numerous, interlaced muscu- 
lar fasciculi which compose the foot. This last varies considerably as to its 
size and form, and can be protruded a long way out through the open shells, 
but may also be wholly withdrawn. 
2 Lingula forms an exception. Here the prin- visceral sac to the valves. With Lingula, there 
cipal adductor is a short, solid muscle, which 
stretches straight across from one shell to the 
other, at their posterior extremity. 
3 This muscular apparatus has been particularly 
described and figured by Owen, and Vogt (loc. 
cit.) with Terebratula, Orbicula, and Lingula. 
With the Terebratula, two pairs of muscles 
arise from each valve. The two anterior ones 
arising from the imperforate valve, are the longer, 
and have their origin back of its centre. After the 
crossing of their delicate tendons, they pass 
through the opening of the valve and terminate in 
the pedancle, together with the two posterior ones 
which are short and fleshy, and which arise at the 
‘base of the hinge. Of those of the perforated 
valve, the two posterior ones only pass to the 
peduncle — the two anterior being attached to the 
‘base of the other valve. With Orbicuda, there 
are two posterior, and two anterior fleshy muscles, 
all of which pass obliquely from one valve to 
the other, while the anterior ones sometimes send 
off fibres to the short peduncle. In the space cir- 
cumscribed by these four muscles, lie four others, 
which are small and interlaced, and extend from the 
are, beside the principal adductor, four pairs of in- 
terlaced muscles, which pass obliquely through the 
centre of the cavity of the valves, and are attached 
by their two ends to the visceral sac. 
4 Owen, loc. cit.; and Vogt, Anat. d. Lingula, 
p. 8, Tab. IT. fig. 16-18. 
1 The foot is absent, particularly with those mol- 
lusks which are fixed to rocks and other solid 
bodies, by a calcareous cement. 
2 Usually one pair of these delicate cords passes 
above, and another below, and are inserted upon 
the valves, quite near the four points of insertion 
of the two adductor muscles. This is so with Ano- 
donta, Unio, Cardium, &c. With Isocardia, I 
have found a third pair of cords inserted upon the 
posterior extremity of the summits of the shell. 
These serve not only as Retractores of the foot, 
but when this last is fixed to some point, draw the 
animal towards it. 
8 The laterally-compressed foot of Anodonta, and 
Unio, arises, by a large base, from the abdomen, 
and has carinated borders. That of Pectunculus 
and Venus, is quite similar, but its free border is 
hollowed by a furrow, and is therefore bi-carinated. 
