THE DEVELOPMENT OF ASTERINA GIBBOSA. 243 



both spaces grow out together underneath the growing lobe of 

 the hydroccele, and thus the radial perihffimal canal itself is 

 formed ; we shall find later that the inner perih^mal ring arises 

 as an outgrowth from the oral end of the axial sinus or anterior 

 coelom, and hence it is marked a' in the woodcut. 



Pig. 53 shows us that the fourth and fifth lobes of the 

 hydrocoele have extended over to the right; this being the 

 result of the tendency of the two ends of the hydrocoele, which 

 have become entirely shut off from the anterior ccelom, to 

 approach one another. We also see from the obliquity of the 

 right posterior ccelom (compare figs. 44—46 with figs. 52 and 58) 

 that the lateral flexure of the body on the stalk has commenced. 

 The flexure in a downward direction cannot be well shown by 

 sections. 



PL XXII, figs. 54—57, are sections of a larva rather older 

 than Stage E. We see that the differentiation of the stalk from 

 the body has been initiated by the dorsal constriction of the neck 

 of the prseoral lobe. In consequence of this the anterior ccelom 

 becomes divided into a stalk portion a, and a body portion a', 

 the latter forming the axial sinus. We see, further, that the 

 ventral horn of the left posterior ccelom I'p'c' has pursued its 

 growth, extending obliquely to the right under the gut, and then 

 upwards in a dorsal and anterior direction, and on its course 

 the last of the five arm rudiments appears, viz. V. Fig. 57 

 shows the outgrowth of septa destined shortly to close the 

 ventral communication between this right horn of the left 

 posterior ccelom and the anterior ccelom. The primary lobes 

 of the hydrocoele have each by this time given rise to two 

 lateral lobes, the rudiments of the first tube-feet, the primary 

 ones themselves being destined to form the terminal tentacles 

 of the water-vascular system. 



Figs. 58 and 59 represent a larva about midway between 

 Stages E and F. We see the final division of the hydrocoele 

 from the anterior ccelom, the last connection being in the 

 neighbourhood of lobe 3, and also the separation of the axial 

 sinus from the stalk ccelom. We see also the remains of the 

 larval oesophagus (te.), which already in Stage E has broken off 



