xvi ECHINODEKMATA 471 



eventually come to occupy a position at the posterior pole of the 

 larva. At the same time other changes are occurring. The hydro- 

 coele becomes more and more grooved off from the anterior coelom. 

 The stone-canal is changed from a groove into a tube by the meeting 

 of its edges. As the hydrocoele becomes grooved off it exhibits a 

 dorsal horn and a ventral horn, and a central piece where it remains 

 for a time in connection with the anterior coelom. The ventral 

 horn extends round underneath the larva to the right side, in fact it 

 grows parallel with the left posterior coelom. 



Thus the metamorphosis of the larva may be roughly summed up 

 as consisting in the preponderant growth of the organs of the left 

 side as compared with their antimeres on the right side (right hydro- 

 coele and right posterior coelom), together with the gradual atrophy 

 of the prae-oral portion of the larva which forms the stalk. The 

 lobes of the hydrocoele are the rudiments of the radial water-vascular 

 canals of the adult star-fish, and the completed arm consists of the 

 process o'f the aboral disc, into which an outgrowth from the left 

 posterior coelom extends, and to which the process of the hydrocoele 

 becomes applied. There is little doubt that the process of the aboral 

 disc, with its contained coelom, is to be regarded as an outgrowth of 

 the body, secondarily developed, in order to give support to that long 

 hydrocoele lobe, or radial canal, which was originally a free tentacle. 

 Each primary lobe of the hydrocoele develops lateral lobes in pairs, as 

 branches; of these two pairs are formed before metamorphosis is 

 complete. These are the rudiments of the paired tube feet. 



The adult stomach appears, we have seen, as an outgrowth from 

 the larval stomach on the left side, in the region where the peri-oral 

 coelom has made its appearance as an outgrowth from the left 

 posterior coelom. As the new stomach grows out the peri-oral coelom 

 extends round it ; whilst outside it the left posterior coelom, whose 

 dorsal and ventral horns meet and fuse with one another, forms an 

 outer ring. At the same time short pouches grow out from the larval 

 stomach into the developing arms. These pouches are the rudiments 

 of the pyloric caeca of the adult, and the larval stomach becomes 

 the pyloric sac, whilst the adult stomach is really the eversible 

 sac which the star-fish wraps round its prey. The adult mouth is 

 formed by the fusion of the adult stomach with the ectoderm. 



Only when the metamorphosis is nearly complete does the 

 rectum make its appearance. It grows out in the mesentery 

 separating the right posterior coelom from the left posterior coelom, 

 in the same mesentery, that is to say, in which the larval intestine 

 was situated, and, as mentioned above, Gemmill has shown that it is 

 formed from the stump of the larval intestine. The adult anus 

 appears later still ; it is eventually perforated in the right dorsal 

 inter-radius, using the word " right " in reference to the sagittal 

 plane of the larva. 



The shrinkage of the prae-oral lobe is largely due to the change in 

 form of the ectoderm cells covering it they change from a flat to a 



