THE LAMP SHELLS 3399 



At the present time, the most widely spread group are the 

 Terebratulidce or perforated Brachiopods, to which Terebratulina, 

 here figured, belongs. In all species of this family, the dissimilarity of the 

 two shell valves is strongly marked, one valve being larger than the other, more 

 concave, and perforated at the beak. The hole through the beak in this family 

 resembles in some cases the hole for the wick in an ancient lamp, and has thus led 

 to the name lamp shells being applied to all Brachiopods. Through the perfora- 

 tion a short sinewy stalk emerges, by means of which the animal attaches itself to 

 submarine objects. The hinge at the beak consists of a pair of teeth situated on 

 the larger valve, and fitting into depressions in the smaller valve. Thus, although 

 these shells do not possess an elastic band or ligament like that which binds 

 together the shells of a mussel, they are prevented from falling apart. The valves 

 are shut and opened by means of muscles. In consequence of the position of the 

 animal and of its organs, the larger more concave valve has been called the ventral 

 valve, and the smaller the dorsal valve or lid. The most remarkable feature in 

 these ancient forms is the looped calcareous framework attached to the under 

 surface of the lid near the hinge, and running forward toward the gape of the 

 shell. The differences in the forms of this framework are used for distinguishing 

 the families and other subdivisions of the Brachiopoda. Its form and extent can 

 be made out in well-preserved fossil specimens. These two spirally coiled append- 

 ages are the "arms," from which the class takes its name, and deserve a little closer 

 description. The illustration given represents the under surface of a valve of the 

 serpent-headed Terebratula, near the back of which the mouth opens (p). The 

 arms rise on each side of the mouth, run forward, bend 

 back again, to end in an elegant coil. We may compare 

 them to the crown of tentacles and other organs associ- 

 ated with the mouth of many worms, and of the moss 

 animals, only here they are stiffened by a calcareous 

 skeleton. As can be gathered from their rigid calcareous 

 frame, they are capable of only slight movement, and 

 even their fringes are more or less stiff. As to the 



functions of these arms, inasmuch as they are traversed 



, . , BACK VAI,VB OF Terebratuhna, 



by canals, and covered with cilia, we are justified m SHOWING THE ARMS. 



assuming them to be gills. The animals feed on fine 



particles brought to the mouth by the streams of water set up by the cilia on these 

 gills. In only one genus (Rhynconella} , belonging to the family of the Rhynconellidee ; 

 do the arms themselves project beyond the shell to seize prey. The alimentary 

 canal is short, and ends blindly at x. The body of the animal within the shell is 

 enveloped in two mantle folds closely applied to the shell; the latter being formed 

 by a secretion from the outer surface of this mantle. Very simple reproductive 

 organs lie in vessel-like widenings of these mantle folds. The sexes are separate, 

 and may sometimes be recognized by differences in the form of the shell. A pair 

 of membranous funnels, internally ciliated, serve as ducts for the genital products; 

 the free ends of these funnels open into the body cavity, and conduct the reproductive 

 elements outward. The resemblance of these funnels to the nephridia of worms, 



