Phylum Echinodermata and Related Groups 555 



that of the chordates, and this represents the only phylum among the 

 invertebrates in which such a condition is found. The head region is 

 flattened, the trunk region somewhat ovoid, and the tail rounded ; thus the 

 animal is distinctly streamlined. The five fins present in the trunk and 

 tail region are flattened in a horizontal plane : one pair of fins is located 

 on the trunk, the second pair on the posterior part of the trunk and 

 anterior portion of the tail, and the caudal fin is unpaired and at the 

 terminal portion of the tail. These fins do not function independently, 

 but act as balancing organs and aid in swimming movements. The ani- 

 mal swims by contractions of the longitudinal muscles of the body wall. 



OVABY FIN -OVltXJCT 



Fig. 184. — Ventral view of the arrow worm Sagitta. 



The head region is highly modified and has many specialized struc- 

 tures for the capture of smaller animals and plants which constitute the 

 food of the arrow worms. The mouth itself is recessed within a depres- 

 sion known as the vestibiilum. The shape and size of this depression 

 may change with the size of the prey seized. The mouth is somewhat 

 ventral in position and dorsal to it are two sets of chitinized teeth. The 

 name of the phylum is derived from the long curved sharp rods on either 

 side of the head. These rods are composed of chitin and number eight to 

 twelve on each side. They are of use in capturing prey. Dorsally the 

 head has a pair of complex eyes. 



The coelom is large and divided by septa into the three main re- 

 gions. The digestive system consists of the mouth which leads into the 

 esophagus, and the straight intestine which terminates in the anus at 

 the end of the trunk. Two esophageal diverticula project backward, 

 while two intestinal ones project forward. The nervous system includes 

 a pair of cerebral ganglia above the esophagus, a pair of connectives 

 around the esophagus, and a ventral ganglion in the trunk region which 

 has nerves going to various parts of the body. There are two types of 

 sense organs whose function is fairly clear : the eyes and small tactile 

 papillae which cover the body surface. Other sense organs undoubtedly 

 also are present. There is no special excretory, respiratory, or circu- 

 latory system. 



