DEVELOPMENT. 



145 



The stone-canal arises as a groove along the anterior face of 

 the posterior wall of the anterior coelom. The central 

 portion of this groove be- 

 comes closed to form a 

 canal which opens at its 

 posterior end into the 

 hydrocoel between lobes 1 

 and 2 and at the other into 

 the anterior coelom, which 

 as we have seen opens to the 

 exterior by the primary 

 water-pore. Save for this 

 communication the anterior 

 coelom and the hydrocoel 

 become completely separate. 

 The anterior coelom persists 

 into the adult as the axial 

 sinus. When the primary 

 water-pore becomes con- 

 verted by subdivision into 

 the numerous pores of the 

 madreporite of the adult, 

 most of these are found to 

 lead directly into the stone-canal, but some open into the axial 

 sinus. The stone-canal retains its opening into the axial sinus 

 (anterior coelom). 



The left posterior coelom undergoes a very complicated development. 

 It gives off dorsal and ventral outgrowths which grow on to the right 

 side of the larval body, and a diverticulum which extends round the 

 oesophagus and is called the oral coelom. These all become indistinguish- 

 able in the adult and merely persist as part of the perivisceral cavity. 

 In addition to these the left posterior coelom gives off four interradial 

 prolongations, each of which bifurcates to proceed to the adjacent arm 

 rudiments. These are the first traces of the outer perihaemal ring and of the 

 radial perihaemal canals (Fig. 131). There is a fifth interradial coelomic 

 prolongation, viz. that which lies between lobe 1 and 2 of the hydrocoel and 

 is distributed to arms 1 and 2 ; this lies in the interradixis of the axial 

 sinus and stone-canal and is an evagination of the anterior coelom. Lastly 

 the aboral sinus is an outgrowth of the left posterior coelom, from which 

 it becomes completely cut off in the adult (p. 146). The right posterior 

 coelom remains much smaller than the left. It persists in the adult, as 

 the part of the body-cavity (sometimes called epigastric) which lies between 

 the gut and the aboral body wall of the starfish. It is prolonged into each 

 arm as the space between the two mesenteries of each pyloric caecum, 



Z III L 



FIG. 101. Longitudinal horizontal section of a 

 larva of Asterina gibbosa showing the origin 

 of the hydrocoel and the relation of the 

 coelomic sacs (after MacBride). a anterior 

 body-cavity in the preoral lobe ; Ipc left pos- 

 terior coelom ; Ihy 1. 2. 3 lobes of the hydrocoel, 

 no. 1 being the most dorsal, no. 3 the posterior, 

 no. 5 (not shown) is at the other end of the 

 incipient ring and is the most ventral ; or.coral 

 coelom ; rhy right hydrocoel ; rpc right pos- 

 terior coelom. 



