496 PHILOSOPHY OF ZOOLOGY. 



unnecessary. They are either oviparous, or ovoviviparous. 

 The presence or absence of a shelly covering, furnish cha- 

 racters for a twofold distribution of the groups. 



Section 1. ACEPHALA CONCHIFERA. 



The shell in all cases is external and bivalve ; and 

 exhibits very remarkable differences in the form, relative 

 size, and connection of the valves. The cloak is likewise in 

 the form of two leaves, corresponding with the valves which 

 protect it. 



Order I. BRACHIOPODA. 



Mouth with a spiral arm on each side fringed with fila- 

 ments. 



The genera included in this group constitute the Bra- 

 chiopoda of CUVIER. The lobes of the cloak are free an- 

 teriorly. From the body, between the lobes, the arms have 

 their origin at the margin of the mouth. These arms are 

 capable of folding up spirally. All the species are perma- 

 nently attached to foreign bodies, and inhabit the sea. 

 Their nervous and reproductive systems have received but 

 little elucidation. 



1st Subdivision. 



Shell supported on a fleshy peduncle. 



54. LINGULA. Valves equal, the apex of both attached 

 to the peduncle. 



The peduncle is nearly cylindrical, cartilaginous, and 

 covered with a membrane consisting of circular fibres. The 

 valves are oval, flat, and destitute of teeth, or elastic liga- 

 ments. The adductor muscles are numerous, obliquely 

 placed, and appear capable of giving to the valves a con- 

 siderable degree of lateral motion. The cloak is thin, and 



