440 GENERAL ZOOLOGY 



not ciliated and lacks both spines and pedicelluriae. Regenera- 

 tion is well marked. 



Examples: Holothuria marmorata, trepang, used as food by 

 the Chinese ; Cucumaria frondosa, sea cucumber of the Maine 

 coast; Thyone briareus a common Woods Hole species of sea 

 cucumber (Fig. 262) ; Leptosynapta inhaerens, a wormlike form, 

 without ambulacral feet. Atlantic Coast. 



Fig. 262. — Thyone, a sea cucumber. The branched tentacles (at the right) 

 are alternately drawn into the mouth, wiped off and extended. Ambulacra are 

 present over the surface of the body. 



PHYLUM 17— CHORDATA 



The Chordata are characterized by (1) the presence at some 

 time in the life history of a notochord, a flexible fibrous elastic rod, 

 lying between the central nervous system and the digestive tract ; 

 (2) a tubular central nervous system lying dorsal to the digestive 

 tract; and (3) a pharynx with gill slits, which may or may not 

 function as repiratory organs in the embryo or in the adult. 

 SUBPHYLUM 1. ENTEROPNEUSTA. Unsegmented worm- 

 like animals with three body divisions: (1) proboscis, (2) collar, 

 and (3) trunk (Fig. 263). A blind tubular extension from the 

 pharynx into the proboscis is taken to represent the notochord. 

 The nervous system consists of (1) a dorsal tubular portion 

 lying in the collar, and (2) a layer of nerve fibers beneath the 

 entire ectoderm. The dorsal portion develops from a tube, as 

 in vertebrates. The mouth is ventral and in front of the collar. 

 The pharynx, perforated with gill slits, leads to the intestine, 



