SHALLOW-WATER STARFISHES 9 



to leave a considerable cavity beneath it, in which the genital orifices 

 and respiratory papulae are situated. This membrane is perforated 

 by small pores for the ingress of water, and normally has a large, 

 valvular, central oscule through which the water escapes. The mem- 

 brane has muscular fibers for contraction. 



The eggs are laid and fertilized in the gonocodium, and when the 

 young are matured they often, or perhaps always, escape by rup- 

 tures of the membrane, which later heal up — a sort of natural Caesa- 

 rian operation. 



In this group the eggs are few in number, as usual in all cases 

 where the young are well cared for. Several species of this family 

 occur on the west coast. (See pi. xxxii.) 



SENSES; INSTINCTS; MEMORY; ACQUIRED HABITS. 



Comparatively little is known, from actual observation, as to the 

 nature or aaiteness of the senses in starfishes. 



At the tip of each ray, in the apical plate, there is a small and 

 very simple eye, or pigmented eye-spot, often red in color during 

 life. It is at the end of the radial nerve. Probably this kind of an 

 eye is of use only for distinguishing different degrees of light, or 

 possibly the shadow of a nearby moving object. 



Close to the eye there is a single odd ambulacral tube, generally 

 considered a tentacle, and supposed to have, at least, the sense of 

 touch. However, all parts of the integument, and especially the 

 papulae and sucker-feet, are also very sensitive to touch. There- 

 fore it may be that these tentacles have some other sense func- 

 tion more developed than elsewhere, possibly that analogous to 

 smell or taste, for appreciating the purity or salinity of the water. 

 They must have a sense for heat and cold, as this determines their 

 migrations at certain seasons. That they are somewhat sagacious 

 and discriminating in their successful selection of the young oysters 

 for food in spite of the labor and time required to open them, is 

 good evidence that they have some sense analogous to taste, and have 

 sense enough to follow it. 



That they can be attracted to an open oyster or other favorite 

 food, from some distance, indicates that they have some sense simi- 

 lar to smell, like that of many mollusca and Crustacea that are easily 

 attracted by a dead fish or other odorous food. As they have no 

 proper head or central brain, but only a circle of equal nervous 

 centers, it is clear that they can have no consciousness of right or 

 left, forward or backward. Each ray is practically equal to every 



