476 



INVERTEBEATA 



CHAP. 



become hollowed out ; but Gemmill has shown that this is an error 

 and that these spaces originate in Jsterias in the same manner as 

 they arise in Asterina. 



As metamorphosis proceeds the prae-oral lobe shrinks more and 

 more, and the neck of the lobe becomes constricted. The effect of 



Fig. 366. — Longitudinal frontal sections of larvae of AsUHna gibbosa seven to eight days 

 old, to show the beginning of the metamorphosis. (Original.) 

 A, section througli a larva in the dorsal region of the body. B, section tlirough a larva near the 

 median region of the body. C, section through a larva in the ventral region of the body. Letters as 

 in previous figure. In addition, p./i, rudiments of perihaemal spaces ; j)./i.l.2, rudiment of iierihaemal 

 space intervening between lobes 1 and 2 of the liydrocoele; p.h.2.S, rudiment of perihaemal space 

 between lobes 2 and 3 of the hydrocoele, and so on ; st.c, stone-canal (still an open groove). 



this is to bring arm No. 5 closer and closer to hydrocoele lobe 

 No. 1 (Fig. 368). At the same time each hydrocoele lobe gives rise 

 to two pairs of lateral branches springing from its base. These are 

 the rudiments of the paired tube feet (Fig. 368, t.f), while the tip of 

 the primary lobe forms the azygous tube foot in which the radial canal 

 terminates. 



