414 GENERAL ZOOLOGY 



opens into a U-shaped digestive tube terminating in an anus, also 

 located in the center of the lophophore but separated from the 

 mouth by a small lobe, the epistome, overhanging the mouth. 

 The digestive tube is supported in a body cavity by longitudinal 

 mesenteries. The body cavity is divided by a transverse portion 

 into two parts, of which the anterior one is continuous with the 

 cavities in the epistome and the lophophore. There is a blood 

 circulatory system and also a nephridial system consisting of a 

 pair of tubes leading from the body cavity to the outside. The 

 nervous system consists of a ring about the anterior end of the 

 alimentary canal and nerves extending from it to the tentacle and 

 other parts of the body. The animals are monoecious. Devel- 

 opment includes a larval stage (actinotrocha) . There are only a 

 single genus and about 11 species (Pratt). 



Example: Phoronis architecta, lives in a straight tube, about 

 12 cm. in length. Coast of North Carolina. 



PHYLUM 12— CHAETOGNATHA 



The Chaetognatha (bristle-jawed) are free-swimming, uncili- 

 ated, and unsegmented marine worms. The group gets its name 

 from a fringe of prehensile, hooklike bristles and one or two rows of 

 small spines on either side of the mouth. The digestive tract 

 is a straight tube lying in a body cavity to which it is attached by 

 dorsal and ventral mesenteries. An anus is present. The body 

 cavity is divided into three parts by transverse septa. Special 

 organs of circulation, excretion, and respiration are absent. A 

 large dorsal nervous ganglion is located anteriorly above the 

 mouth and a ventral ganglion under the intestine. There are a 

 pair of eyes and an unpaired olfactory organ. These animals are 

 monoecious and their development is interesting because the 

 coelomic cavity is formed by the longitudinal fusion of a pair of 

 diverticula of the lining of the archenteron, much as in annelids 

 and chordates. 



Example: Sagitta elegans, the arrowworm, about 3 mm. in 



length and common in the North Atlantic. 



PHYLUM 13— ANNELIDA 



The Annelida (ringed or annulated forms) are the segmented 

 worms, which are found in fresh and salt water and in the earth. 

 They differ from other worms in the fact that the body is made 



