ASTEROIDEA 623 



N. alata (Pourtales). Twenty arms present; centrodorsal with 

 about 30 cirri in 2 irregular rows, each with 15 to 18 segments: West 

 Indian waters, from 78 to 310 fathoms. 



2. COMACTINIA A. H. Clark. Segments of cirri without dorsal 

 processes ; 10 arms present : 3 or 4 species ; in West Indian waters. 



C. meridionalis (E. C. and A. Agassiz). Length and stoutness of 

 arms variable ; segments of genital pinnules very short : Beaufort, N. C, 

 and southwards, in 12 to 100 fathoms. 



Class 2. ASTEROIDEA.* 



Starfish. Echinoderms in which the radii are more or less elongated 

 so that the body has the form of a five-armed star (Fig. 965, A). In 

 some of them the number of arms is normally greater than five, and 

 many abnormal individuals are found with more or sometimes less than 

 this number. The oral surface is directed downwards and on it the 

 animal rests and moves about; the aboral surface is directed upwards. 

 Surrounding the mouth is a membranous area called the peristome 

 (Fig. 966, A, 3). The main axis of the body is short and extends from 

 the mouth in the middle of the oral to the middle of the aboral surface; 

 the anus has an excentric position in this surface. 



Extending from the peristome along the oral surface of each arm 

 is the median ambulacral groove, from the walls of which either two or 

 four rows of ambulacral feet project into the surrounding water (10). 

 On the aboral surface in an interradius is the 

 madreporite (9), the external opening of the am- 

 bulacral system. 



The external epithelium is ciliated. Spines or 



tubercles appear on the surface of all starfishes; ^^^ gg^ ^ ^^^ g^ 



some of these especially along the edges of the gf i^^^i^^j?^ {hudwigl 

 ambulacral grooves are movable. Minute pincer- 



like organs called pedicellariae (Fig. 969, A and B), which are 

 modified spines, are usually present; they may be either sessile 

 or pedunculate (stalked), and their jaws are either straight or 

 crossed like scissors. Paxillae (C), also modified spines, are present in 

 some groups, each paxilla being a thick calcareous rod, the summit of 

 which is covered with minute spines. Short finger-like projections of 

 the body wall called papulae (Fig. 966, 15) also appear, especially on the 



* See "North American Starfish," by A. Agassiz, Mem. Mus. Corap. Zool., Vol. 5, 

 1877. "Report on the Asteroidca," Zoology of the Challenger, Vol. XXX, by W. P. 

 Sladen, 18S9. "Asteroidea," by A. E. Verrill, Amer. Jour. Sci., Vol. 49, 1895. 

 "Revision of Certain Genera and Species of Starfish," by A. E. Verrill, Trans. Conn. 

 Acad., Vol. 10, p. 145, 1899. "Asteroidea of the North Pacific and Adjacent Waters," 

 by W. K. Fisher, Bull. U. S. N. M., 1911. 



