338 ECHINODERMATA [CH. 



connected with a pillar of similar cells running alongside the stone- 

 canal which used to be called the heart, under a mistaken idea of its 

 function, but which we shall term the genital stolon. The genital 

 rachis is formed as an outgrowth from the genital stolon and the 

 latter is an outgrowth from the coelomic wall, so that the genital 

 cells are derived from the coelomic cells as in other Coelomata. 

 The genital stolon is interposed between the general coelomic cavity 

 of the animal and a special division of the same which is called 

 the axial sinus, and which runs parallel to the stone-canal. 

 The axial sinus is derived from the anterior portion of the 

 coelom in the larva. Underneath the madreporite there is still 

 another division of the coelom completely shut off from the rest, 

 which maybe termed the madreporic vesicle. It seems to be 

 derived from a sac in the larva which is contractile; it possibly 

 represents the pericardium of Balanoglossus (see p. 389). The 

 genital stolon projects into the axial sinus; it has a brown colour 

 which no doubt suggested the connection with the blood-system 

 to the earlier anatomists, but true blood-vessels do not exist in 

 Echinodermata. The ova and spermatozoa are thrown out into the 

 water by pores situated on the under or oral surface at the base of 

 the arms and unite with each other there. The young lead a free- 

 swimming existence, and are so unlike the star-fish that no one would 

 ever dream of suspecting that the two had anything to do with 

 each other. As however these peculiarities are fundamentally the 

 same in each of the groups of the Echinoderms they will be dealt 

 with later when the characters of these other groups have been 

 studied. 



The other species of star-fish, which are all grouped together in 

 the class Asteroidea (Gr. d<myp, a star ; eTSos, shape), differ but little 

 in really fundamental points from Asterias rubens. Pedicellariae 

 may, as we have seen, be absent ; the arms may be short so that 

 the shape almost becomes that of a pentagon and the arrangement 

 of the plates and spines constituting the skeleton may vary very 

 much. In one family, the Astropectinidae, there is no anus, the 

 rectum ending blindly, and the tube-feet have pointed ends. These 

 star-fish do not climb but run over the surface of the sand. 



The number of arms is most often five, but not only do indi- 

 vidual variations from this rule occur in species where five is the 

 normal number, but species and even genera and families are 

 characterised by having a larger number: the common Sun-star, 

 Solaster papposus, for instance, has from eleven to thirteen arms. 



