BRACHIOPODA, 251 
Class BRACHIOPODA 
The Brachiopods or Lamp-shells are quaint marine animals, once 
very numerous, but now decadent, The body is enveloped dorsally and 
ventrally by two folds of skin or mantle; these secrete a shell, usually 
of lime, but sometimes organic. The development of this shell has 
apparently modified both the position and the relations of. the organs. 
There is no real resemblance between a Brachiopod shell and that of a 
bivalve Mollusc, except that both consist of two valves, In Brachiopods 
these lie dorsally and ventrally; in Lamellibranchs they are lateral ; 
moreover, in Brachiopods the ventral 
valve is usually the larger. It is hardly 
necessary to say that the Brachiopod 
organism is not the least like a Mollusc. 
A considerable part of the space 
between the valves of the shell is filled 
up by two long ‘‘arms,” which are 
coiled in a spiral, and often supported 
by a calcareous skeleton. These arise 
in development from the specialisation 
of a horseshoe-shaped ‘‘lophophore,” 
such as is characteristic of the Polyzoa. 
The mouth is placed between the arms, 
and opens into the ciliated food canal. 
This may end blindly, or may be 
furnished with an anus placed near Fic. 130.—Interior of 
the mouth ; in Cyrazza the anus is dorsal Brachiopod shell, showing 
and posterior. The muscular system calcareous support for the 
is well developed, the shell being both “arms.” —After Davidson. 
opened and closed by means of muscles. 
There is a nerve-ring round the gullet, with a slight brain and an 
inferior ganglion. Sensory structures in many cases perforate the 
valves. Above the gut lies the heart, which is connected with blood 
vessels. Two (or more rarely four) nephridia open. near the mouth, 
and serve also as genital ducts. The posterior region of the body often 
forms a stalk by which the shell is moored, but in many this stalk is 
absent, and the animal is directly attached to the substratum. The 
sexes are sometimes separate, but perhaps some are hermaphrodite. 
There is a metamorphosis in the development, and the larvee resemble, 
in some respects, those of Polyzoa. 
The Brachiopods date from the earliest known fossiliferous rocks, and 
had their maximum representation in the Ordovician and Silurian, 
