192 THE ACEPHALA. §$ 175, 176. 
in some species covers even the whole of the siphon.© Very often, how- 
ever, this epidermis is worn away upon old portions of the shell, which 
is quite striking with those which have lamelliform or pilous prolongations 
around the borders of the shell. 
The two shells are joined together partly by a hinge (Cardo), and partly 
by an elastic tissue (Ligamentum). This last, either external or inter- 
nal, is antagonistic to the adductor muscles of the shell. It is composed 
of elastic fibres, the internal of which, when the shell is closed, are com- 
pressed between the borders of the hinge, while those which are external 
are lengthened out. In both cases, their natural action is to open the two 
shells.¢ 
§ 175. 
The Terebratulae havea very remarkable internal calcareous support situ- 
ated upon the inner surface of the two shells. It consists, first of two delicate 
outwardly curved peduncles, which arise from the sides of the two cardinal 
teeth situated upon the non-perforated valve ; then there are two other pe- 
duncles which are shorter, and arise from a longitudinal ridge upon the 
centre of the same valve ; these pass in front and unite in an arcuate man- 
ner. The two branches thus formed are abruptly recurved after a short 
course, and unite, forming a.common arc behind the centre of the shell. 
With many, this structure is much more simple, consisting only of a median 
apophysis, from which pass off two alar prolongations which are curved at. 
their extremity.” This structure serves principally for the insertion of 
the tentacles.” ' 
CHAPTER II. 
MUSCULAR SYSTEM AND ORGANS OF LOCOMOTION. 
§ 176. 
The raouscles of the Acephala are composed of simple, smooth fibres. 
8 With Mya, and Lutraria, the epidermis forms 
acomplete sheath around the siphon. 
9 Mytilus hirsutus, Arca barbata, lacerata, 
and ovata. 
J0 I must omit a description of the various forms 
of the hinge and ligament, for they belong properly 
to the department of Zoology. The hinge is wholly 
wanting with the Inclusa, and the ligament is ab- 
sent with the Brachiopoda ; and with Orbicula, 
and Lingula, both are absent. 
‘The Aspergillina are distinguished from all the 
other Acephala, by a singular disposition of their 
valves. . Their mantle ceases early to secrete the 
maiter for the shell-formation. The two valves are. 
then joined at a point, where, most probably, they 
would have been articulated, while the mantle, 
which has only two small openings, and its long 
siphon with a double canal, is covered with a calca~ 
reous formation which constitutes, with the twa 
united valves, the singular tube of these animals. 
1 The ligament is internal with Pecten, Spon- 
dylus, Mya, Lutraria, and Pholas ; but external 
with the Chamacea, Cardiacea, Arcacea, and Nai~ 
ades. It is half external and half internal with. 
Mailleus, and many other species. 
1 Terebratula chilensis, dorsata, dentata, and 
Sowerbyi ; see Owen, hoc. cit. Pl. I. fig. 4. 
2 Terebratula rubicunda, and psittacea. 
8 According to Owen, in those species which 
have this apparatus highly developed and bent 
backwards, these arches, notwithstanding their cal- 
careous nature, are somewhat elastic; and when 
the valves are closed, they are slightly depressed, 
and thus may serve in the absence of the elastic 
ligament, for the opening of the shell. 
