BRACK rOPODA. 125 



CLASS BRACHIOPODA. 



These headless niolkisks are enclosed in bivalved shells, which 

 are equal on either side of a vertical line let fall from the beak 

 (equilateral), while the valves are almost always unequal (iyequi- 

 valved). The larger one is called the ventral, and the smaller the 

 dorsal. While in the Lamellibranchs one valve is applied to the 

 right side and the other to the left side of the animal, in this 

 class, one valve is applied to the back, the other to the belly of 

 the animal. The ventral valve has generally a prominent notched 

 or perforated beak, through which passes, in most species, a 

 pedicel or byssus to attach the animal to some foreign body, for 

 bracbiopods are deprived of the power of locomotion. JN^o living 

 species are free. The one or two accessory pieces occupying a 

 triangular opening under the beak, form an area called the delti- 

 dium ', the form and structure, the presence or absence of this, 

 and the muscular impressions, afford good generic characters. 

 The shells are perforated vertically by canals, which connect the 

 inner and outer surfaces. The animal has usually two long spiral 

 prehensile arms developed from the sides of the mouth, whence 

 the name of the class. In many bracbiopods these "arms" are 

 supported by a more or less complicated framework. It is claimed 

 by some eminent embryologists that bracbiopods belong with the 

 articulates, near the annelids, and some later systematists remove 

 them from the raollusks and place them with the worms. (See 

 page 90). Unlike the polyzoa they are never composite. Bracbio- 

 pods, of all the mollusks, enjoy the greatest range in climate, 

 depth and time. Tliey are all marine, and mostly inhabit the 

 deep sea, so that less than 100 living species are known. They 

 are among the oldest of existing forms of animal life. Over 

 4,000 extinct species have been described, distributed through all 

 rocks of marine origin from the Cambrian upwards. The}" 

 attained their maximum (both of generic and specific develop- 

 ment) in the Silurian age, and are the most numerous fossils of 

 the Silurian deposits. The hingeless genera (as Lingula) are 



