156 



PHYLUM ECHINODERMATA. 



continuity with one another, from the rudiment of the intestine. 

 Soon after this has happened the anterior coelom separates 

 from the hydrocoel, which, placed between the enteron and 

 the ectoderm on the left-hand side of the body (Fig. 113), at 

 once acquires the characteristic horseshoe form. The anterior 

 coelom then acquires its external opening, the water-pore, 

 which is placed on the left side of the body just in front of the 

 fourth ring of cilia. Later the hydrocoel develops an anteriorly 



directed outgrowth which 

 acquires an opening into 

 the anterior coelom and 

 forms the stone-canal. 



The posterior coeloms 

 now shift ; the left-hand 

 one moves posteriorly and 

 comes to lie like a cup 

 over the hind end of the 

 enteron (Fig. 113, 5), while 

 the right sac extends ante- 

 riorly and following the 

 enteric wall reaches on to 

 the left side (Fig. 114). 

 The right posterior coelom 

 gives off from its anterior 

 end five forwardly directed 

 diverticula (Fig. 113, 2). 



FIG. 113. Longitudinal- vertical section of a free- TV^^ VP>pnmp nut nff from 

 swimming larva of Antedon..^ 1 stem- joints ; J 



at a later stage, and 

 rise to the chambered 



coelom 



organ. 



The calcareous plates make their appearance in the embryo 

 on the sixth day. They are shown in Fig. 114, which however 

 is taken from a larva after attachment. 



There are five orals (Fig. 114, or] arranged in a horseshoe 

 curve near the posterior end. The horseshoe is set obliquely to 

 the long axis, its dorsal end being posterior to its anterior end, and 

 the open end of it is directed ventrally. Parallel to this row, but 

 anterior, are the five basals (ba) set in a similarly disposed horse- 

 shoe curve. Both the orals and basals are, as shown by the 

 later development, interradially placed. In front of the basals 



