144 PHYLUM ECHINODEBMATA. 



The early development and formation of the larval mouth and 

 anus (p. 135) have already been dealt with. The enterocoel or 

 coelom has the form of an anterior unpaired diverticulum of the 

 enteron extending into the preoral lobe and sending back two 

 lateral prolongations, one on each side of the larval gut (Fig. 98). 

 This soon separates from the gut, and the posterior prolonga- 

 tions of it eventually meet one another ventrally, their opposed 

 walls forming a ventral mesentery. The primary water- pore is 

 now formed as a pocket of the unpaired portion of the coelom. 

 It meets the skin of the dorsal surface just to the left of the 

 middle line, and a perforation is formed at the point of contact. 

 At the same time there is formed, first on the left side of the 

 enteron and then on the right, a septum, by which the hinder 

 parts of the two coelomic prolongations referred to above 

 become separated off from the anterior part and form closed 

 sacs called the right and left posterior coeloms respectively. 

 The unpaired portion of the coelom, together with the anterior 

 portions of the two prolongations, constitute the anterior coelom 

 of the larva into which opens the primary water-pore. 



The hydrocoel is developed as an outgrowth from the hinder 

 end of the anterior coelom on the left side, and while it is yet 

 but faintly marked indications of its five primar} 7 lobes appear. 

 These, which are numbered* 1, 2, 3, 4, and 5, No. 1 being the most 

 dorsal, are arranged in a curve open anteriorly, the hydrocoel 

 sac assuming a horseshoe shape (Figs. 101, 102). The two limbs 

 of the horseshoe eventually come together, enclosing the stalk 

 of the preoral lobe and the oesophagus of the adult, when that is 

 formed on the left side of the larva. In this way the circumoral 

 water-vascular vessel is developed from the horseshoe-shaped 

 hydrocoel. The radial canals are developed from the lobes of 

 the hydrocoel. Shortly after the formation of the left hydrocoel 

 or hydrocoel proper, a rudimentary right hydrocoel is developed 

 as an outgrowth from the hinder wall of the anterior coelom ta 

 the right of the middle line (Fig. 101, rhy). It loses its connexion 

 with the anterior coelom and persists into the adult as a 

 small thin-walled sac beneath the madreporite. 



* The relations of these numbers to the enumeration adopted for the 

 adult is explained on p. 120. The numbering here adopted has relation 

 to the fact that the hydrocoel before it closes to form a ring has two ends,, 

 one of which may be called anterior. 



