190 



COMPARATIVE ANATOMY. 



159. 



Thus, then, an organism is developed, the antimeres of which 

 present themselves as the radially-arranged " arms ;" each of these 

 arms has primitively the value of one person. By the concrescence of 

 these persons an individual of a higher order, an animal colony, is 

 formed. In the Asteroida the number of arms is not definitely 

 fixed; in some there is a large (14 in Solas ter), in others (Astera- 

 canthion) a smaller number (4). In most the number is limited to 

 five, and the typical number, which predominates in other Echino- 

 derma, is fixed at this. The number of arms in Brisinga varies from 

 0-12. The point at which all the arms are connected forms the 

 common body in the Asteroida ; it carries the mouth. This lies on the 

 ventral surface, which is oral, and has an aboral surface opposed to it. 

 In the arms the ventral surface is distinguished by the possession of 

 rows of expansive and movable processes — ambulacral feet; these 

 form an " ambulacrum " in each arm, and are attached to a depression, 

 which runs along the arm (ambulacral groove). They correspond in 

 character with the metameric arrangement of the arms, which is ex- 

 pressed in other parts. There are four rows in Asteracanthion, and 

 two in most of the other forms. It is uncertain whether these 

 structures are allied to the parapodia of Vermes or not. The ventral 

 surface is also known as the ambulacral. There is no difference in 

 the extent of the ambulacral and antambulacral (dorsal) surfaces. 



The radii or arms vary greatly in the extent to which they are 

 independent of the common body ; in not a few forms they show a 

 tendency to shorten so as to form a body disc, and so, at the same 

 time, the indication of the primitively individual character of the 



Fig. 96. Three forms of Starfishes, A B ( ', in which concrescence and consequent 



loss of individuality in the arms is being gradually effected. All three are 



figured from the oral, or ambulacral, surface of the body. The ambulacra are 



represented by rows of dots, o Mouth, r Eadii (Arms), ir Interradii. 



arms is lost (Oreaster, Pteraster, Asteriscus, etc.). A comparison of 

 the three forms of Sea-stars (Fig. 96, A B C) here figured will 

 make this quite clear. 



