BRACH1OPODA. 



251 



Class BRACHIOPODA 



The Brachiopuds 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 Bracniopod 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 FIG. 

 the mouth ; in Crania the anus is dorsal 

 and posterior. The muscular system 

 is well developed, the shell being both 

 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 larvae resemble, 

 in some respects, those of Polyzoa. 



The Brachiopods date from the earliest known fpssiliferous rocks, and 

 had their maximum representation in the Ordovician and Silurian. 



130. Interior of 

 Brachiopod shell, showing 

 calcareous support for the 

 "arms." After Davidson. 



