134 



SYSTEMATIC ZOOLOGY 



the shell, but in some species can be protruded beyond the 

 valves. In most cases the arms of the lophophore are sup- 

 ported by a calcareous skeleton, which is attached 

 to the dorsal valve of the shell. Within the body 

 itself we find a body-cavity, which contains the 

 simple alimentary canal, nearly straight or coiled, 

 /. in some genera ending blindly, in others provided 



with an anus. There is a simple heart, a pair of 

 excretory organs, and a nervous system consisting 

 chiefly of a nerve ring with ganglionic enlarge- 

 ments surrounding the oesophagus. Organs of 

 special sense are very rudimentary or wanting. 

 Four germ-glands are usually present, some 

 genera being hermaphroditic, others having the 

 sexes separate. In their development larvae are 

 formed which resemble closely some other worm 

 larvae. One point of special interest attached to 

 the Brachiopoda is their great antiquity and the 

 fact that some genera living to-day, such as 

 Lingula (Fig. 124), are likewise found in the 

 Silurian. The fossils are much more numerous 

 than the living species. Brachiopoda are com- 

 mon along the eastern coast of the United States 

 at various depths. 



SUBTYPE VII. ANNELIDA 



The Annelida ( Lat. anulus, ring), the ringed 

 or jointed worms, are the most highly developed 

 animals in this type. The majority of them are 

 readily distinguished from other worms by the 

 ringed appearance of the body, so familiar in the 

 Fk;. 124. Lingula, common earthworm. Each animal consists oi a 



one of the oldest • r i_ • n n 1 



D . . . series 01 parts, techmcallv called segments, or 



Brachiopoda ; nat- ' - b 



uraisize. (Drawn metameres, or somites, placed one behind the 



from specimen.) ,, , 11 n 1 vi 



other, and generally all very much alike except 

 the first and last, which are more or less modified. These rings or 

 segments are not merely superficial markings, as a rule, but cor- 

 respond to an interna] segmentation or repetition of parts. In a 



