640 



CHAPTER XXXIV. 



MOLLUSCOIDEA — continued. 



Brachiopoda. 



Brachiopoda (Palliobranchiatd). — The members of this class are 

 defined by the possession of a body p?-otected by a bivalve shell, which 

 is lined by an expansion of the integument, or "mantle." The mouth 

 is furnished with two long spirally - coiled cirriferous p?'ocesses or 

 " arms" which act as respiratory organs. The nervous system con- 

 sists of an oesophageal ring, upon which infra-oisophageal and supra- 

 cesophageal ganglia are developed. One or tivo pairs of tubular 

 " nephridia " are present, which act as ducts to the reproductive 

 organs. The sexes are distinct or united. 



The Brachiopoda are essentially very similar in structure to the 

 Polyzoa, from which they are distinguished by the fact that they are 

 never composite, and by the possession of a bivalve, calcareous, or 

 sub-calcareous shell. All the living forms, except Lingula (Glottidia) 

 pyramidata (fig. 486) are fixed in their adult condition to some 

 foreign object, but many of the fossil forms seem to have been 

 permanently free and unattached. 



As regards the anatomy of their soft parts, the internal organs in 

 the Brachiopoda are enclosed within two integumentary expansions 

 which constitute the " lobes " of the " mantle," and which are gener- 

 ally regarded as being placed on the dorsal and ventral aspects of 

 the body respectively. The space included between these two flaps 

 of the integument is the "pallial cavity," the viscera and muscles 

 being situated towards the beaks of the shell, and separated from 

 the mantle-cavity by a membranous partition, which is perforated 

 by the aperture of the mouth (fig. 487). The larger part of the 

 mantle-cavity is occupied by two long oral processes, commonly 

 spoken of as the " arms," which are the principal organs of respira- 

 tion, and also serve to bring food-particles to the mouth by the cur- 



