PHYLUM ECHINODERMATA 197 



Respiration. The dermal branchiae (Fig. 133, 5) function 

 as respiratory organs (p. 193). 



The Nervous System. Besides many nerve-cells which lie 

 among the ectoderm cells, there are ridges of nervous tissue, 

 the radial nerve-cords (Fig. 135, n,r.\ Fig. 133, 15), running along 

 the ambulacral grooves, and uniting with a nerve-ring (Fig. 135, 

 nerv.circ) encircling the mouth. The apical nervous system 

 consists of a trunk in each arm which meets the other trunks 

 at the center of the disc; these trunks innervate the dorsal 

 muscles of the arms. 



Sense-organs. The tube-feet are the principal sense-organs. 

 They receive nerve- fibers from the radial nerve-cords. At the 

 end of each radial canal (Fig. 135, /) the radial nerve-cord ends 

 in a pigmental mass (oc.) ; this is called the eye, since it is a light- 

 perceiving organ. The dermal branchiae are probably sensory, 

 also. 



Reproduction. The sexes of starfishes are distinct. The 

 reproductive organs are dendritic structures, two in the base of 

 each arm; they discharge the eggs or sperms out into the water 

 through pores in the aboral surface at the interspace between 

 two adjacent arms. The eggs of many starfishes are fertilized in 

 the water; they are holoblastic (p. 87), undergo equal cleavage, 

 and form a blastula and gastrula similar to those shown in 

 Figure 51, K, M. The opening (blastopore) of the gastrula be- 

 comes the anus, and a new opening, the mouth, breaks through. 

 Ciliated projections develop on either side of the body, and a 

 larva, called a Bipinnaria (Fig. 150, B), results. This changes 

 (metamorphosis) into the starfish. 



Behavior. The starfish moves from place to place by means 

 of its tube-feet (p. 194). During the day it usually remains quiet 

 in a crevice, but at night it is most active. 



The responses of the starfish to stimuli are too complex to 

 be stated definitely. When a starfish is placed on its aboral 

 surface it performs the " righting reaction," i.e. it turns a sort 

 of handspring by means of its arms. Professor Jennings 



