THE STAEFISH SOL ASTER ENDECA. 33 



A great decrease in size on the part of the axial ccelom, with consequent narrowing of 

 the septum, takes place in the later stages of metamorphosis. Next occurs breaking 

 down of the part of the septum which stretches between the enteron and the axial 

 coelom, so that this coelom is now attached only to the body-wall. 



As the whole body expands in growth, the body-wall leaves a deep inward pro- 

 longation or septum (interbrachial septum) in each interradius. The inner portions 

 of the septa become thickened and develop calcareous plates, while the outer portions 

 seem to disappear, leaving the inner ones as isolated pillars or arches stretching across 

 the body-cavity from the oral to the aboral surface. The skeletal plate at the oral end 

 of each pillar forms the so-called odontophore. The axial coelom, with its contained 

 structures, is attached to the inner edge of the interbrachial septum belonging to the 

 madreporic interradius. 



The perforation of the outer portions of the septa referred to above had not taken 

 place in my oldest sectioned specimens, which were six months old. 



The posterior coelom gives origin to the following derivatives : — (1) the general 

 hypogastric coelom ; (2) the pharyngeal coelom; (3) all the segments of the external 

 oral circular sinus except that of interradius IX/I; (4) the aboral circular sinus and 

 the genital rachis. 



Hypogastric Coelom. — By far the greater part of the cavity which in the adult 

 surrounds the enteron and its diverticula is derived from the posterior coelom of the 

 larva. As the two horns of the latter grow round the enteron in the sagittal plane 

 towards the madreporic interradius, they, as well as their parent cavity, begin to 

 invade the right and left sides of the larva so far as other structures allow. However, 

 the early condition here (PI. III. tigs. 28-33) does not warrant our speaking of right 

 and left horns of the posterior coelom as Masterman does in the case of CrihreUa. On 

 the right side the extension of the posterior coelom is checked by the right lateral 

 diverticulum of the anterior coelom (the future epigastric coelom), and on the left side 

 by the left lateral diverticulum, the bottom of which, as we saw before, gives rise to 

 the hydrocoele. 



The walls of the posterior coelom, meeting back to back with the walls of these 

 diverticula, form right and left mesenteries for the enteron, both of which, to begin 

 with, extend dorso-ventrally and are crescent-shaped, with the convexity directed 

 backwards. On the right side the crescent becomes a circle when, during metamor- 

 phosis, the epigastric coelom is cut off from its parent cavity, and the horns of the 

 posterior coelom meet at the madreporic interradius. At first the epigastric coelom is 

 of sufficient size to cover the whole of the right face of the enteron. In the later 

 stages of metamorphosis the enteron expands in the (Starfish) horizontal plane so 

 much that the epigastric coelom no longer covers the whole of its aboral surface, but 

 is confined to a central area around which there is a gradually increasing margin 

 which is covered by an aboral extension of the posterior coelom. As the paired radial 

 VOL. XX. — PAKT I. No. 5. — Felnmry, 1912. f 



