196 THE SCIENCE OF LIFE. 



ward replaced by the vertebral column. From this re- 

 semblance the partisans of evolution have claimed that 

 this simple cellular structure is the prototype of that 

 which distinguishes the higher animals, and that from 

 the simple Ascidian the Vertebrate has been developed. 

 Such foreshadowings of higher types is not uncommon. 

 It will require, however, much greater evidence to prove 

 transmutation than such resemblances. 



4. BRACHIOPODA are protected by a bivalve shell, 

 which is lined by an expansion of the integument, or 

 " mantle." The valves of the shell are applied to the 

 dorsal and ventral sides of the body. The ventral valve 

 is usually larger and more convex than the other ; but 

 they are symmetrical, that is, a vertical line from the 

 hinge divides the shell into equal parts. The ventral 

 valve generally has a hole, or foramen, through which a 

 fleshy foot protrudes for attachment. The mouth is 

 furnished with two long arms, fringed with cirri, gener- 

 ally coiled up and supported by a bony frame-work in 

 the shell the " carriage-spring apparatus." As there 

 are no gills, the animal respires by the arms or the man- 

 tle. Brachiopods were once very abundant, over two 

 thousand extinct species having been described ; but 

 less than one hundred species are now living. 



In all the Molluscoida the nervous system consists of 

 a single ganglion, or of a principal pair with accessory 

 ganglia placed between the oral and anal apertures, or 

 on the ventral surface of the body. Some naturalists 

 connect them with the Worms. 



5. LAMELLIBRANCHIATA (Lat., lamella, a plate ; Gr., 

 bragchia, gill) comprise the ordinary bivalves, as the 



