378 Brachiopoda. MOLLUSCA. Bkachiopoba. 
Class V. — BRACHIOPODA (Brachiopods.) 
Tiik Bracliiopods are so closely allied to the con- 
chiferous molliisks, that they have been considered by 
many naturalists as forming only a simple division of 
that extensive class. “ In all essential points,” says 
Professor Owen, “ the Brachiopoda closely corres- 
pond with the acephalous MoUusca, and we consider 
them as being intermediate to the Lamellibranchiate 
and Tunicate orders ; not, however, possessing, as far 
as they are at present known, a distinctive character of 
sufficient importance to justify their being regarded as 
a distinct class of mollusks, but forming a separate 
group of equal value with the Lamellibranchiata." In 
accordance, however, with the more modern systems 
of classification, as propounded by Dr. Gray, Mr. Wood- 
ward, and the Messrs. Adams, we treat them as a 
separate class, the peculiar structure of the animal en- 
titling them to such distinction. Though this struc- 
ture shows them to be comparatively low in the scale, 
yet they are exceedingly interesting both to the physi- 
ologist and the geologist. The number of recent or 
living species is small, but in former periods of the 
earth’s surface it was very great, and the Brachiopods 
at those times occupied the position now taken by the 
Conchifera. The shell of the Brachiopodous mollusca 
is essentially bivalve. It is symmetrical but inequi- 
valve, and is generally attached to submarine bodies 
by a muscular peduncle which passes through a hole 
iti the beak. This in all probability is a byssus when 
the animal is young, but becomes consolidated as it 
increases in age, and forms at length a permanent 
organ of attachment. The valves, instead of being- 
right and left as in the Conchifera, are superior and 
inferior. As the animal lies upon its back, the lower 
valve covers the back of the animal, and therefore is, 
with refei ence to the body, correctly called the dorsal 
valve ; the upper, covering the ventral surface, receives 
the name of ventral valve. The dorsal valve is the 
smaller of the two in general, and is always free ; the 
ventral valve, on the contrary, is usually the larger, 
and has its umbo produced and perforated for the pas- 
sage of its attaching peduncle. These valves are united 
by a hinge without a ligament, and are articulated by 
two curved teeth, which are developed from the margin 
of the ventral valve and fit corresponding sockets in the 
other. Internally the dorsal valve is furnished with 
a peculiar shelly apparatus, called hy some authors the 
“ apophysary system,” and by others the “ internal 
skeleton, and by collectors the “ carriage-spring.” 
“ The principal part of the internal skeleton,” says 
Professor Owen, “consists of a slender, flattened, cal- 
careous loop, the extremities of which are attached to 
the lateral elevated ridges of the hinge. The crura of 
the loop diverge, but again approximate to each other, as 
they advance for a greater or less distance towards the 
opposite margin of the valve. The loop then suddenly 
turns towards the perforate valve, and is bent back 
upon itself for a greater or less extent in different species. 
.... The arches of the loop are so slender that. 
notwithstanding their calcareous nature they possess a 
slight degree of elasticity and yield a little to pressure, 
but for the same reason they readily break off if the 
experiment be not made with due caution.” (See cut 
annexed — fig. 230 — upjier figure.) This extraordinary 
apparatus is intended for the support of the “ fringed 
Fig 230. 
structure of Brachiopods. 
arms” of the animal, a structure which is peculiar to 
this class of mollusks, and which has obtained for them 
their name.* (See lower figure.) The mouth of the 
Brachiopods is placed on the under part of the cavity 
between the lobes of the mantle. Developed from the 
sides of this mouth we see two long appendages, whicli 
appear to be lateral prolongations of the lips, and cor- 
respond with the lips and labial tentacles of the ordi- 
nary bivalves. These have usually been called arms 
— they are free or united by membrane, and are so 
long that they require to be folded up. In most of the 
species they are twisted in a spiral form, and their 
outer margins are fringed with long cirrhi or filaments, 
by means of which the animal creates a current in the 
water that brings objects of food within reach of the 
mouth. Some conchologists imagine that the animal 
has the power of protruding these oral arms beyond 
tbe shell in search of food ; but this does not appear to 
be the case, as from the fact of their being united by a 
membrane, and supported by the internal skeleton 
described above, they must evidently be confined to 
one position. In some of the genera, however, we 
are told that the ends of their arms are “ endowed with 
a degree of voluntary motion, the movement being 
effected by tbe injection of a fluid into the hollow spiral 
tube, by means of which the coils are separated.” 
* From arm, and ■rovs, foot, these arms having been 
supposed to take the place of the feet in the preceding classes. 
