Elements of comparative anatomy (1878) Elements of comparative anatomy elementsofcompar00gege Year: 1878 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 f
Elements of comparative anatomy (1878) Elements of comparative anatomy elementsofcompar00gege Year: 1878 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 gradual
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