300 



ZOOLOGY 



SECT. 



rudiment HIT alimentary canal, and forms the mouth and oesophagus 

 of the larva. 



The enteroccelic pouches grow round the alimentary canal : in 



Larv.org 



B 



Larv.org . 



\ < i 



FIG. 2s(s. Later stages in the development of the Bipimmiia. of Asterina gibbosa A, 

 newly hatched larvu, ventral surface with the beginning of the larval organ at the anterior 

 end and with the larval mouth. B, dorsal half of an embryo of the same age as A. C, some- 

 what older larva with larger larval organ, the ectoderm of the left side removed to expose the 

 alimentary canal and the walls of the body-cavity, arch, archeiiteroii ; hi. p. blastopore ; ect. 

 ectoderm'; ,>t. enteroccele ; l<t,-r. mo. larval mouth; Jaw. orj/. larval organ. (From Ziegler's 

 models.) 



front they unite to form a common cavity, behind they remain 

 separated by a thin partition or septum. The left enterocoelic 

 pouch gives rise now to the rudiment of the ambulacral system 



Fii.. L'x'. Bipinnariaof Asterina gibbosa. A. Diagrammatic lateral view; the alimentar 



canal dotted, the ambulacral system striated, the ectoderm shaded. B. Hipinn.-iria seen from 

 the left as an opaque object, the body -wall of the left side removed. Ilydroccele separated 

 off from left enteric sac and partly surrounding oesophagus, tdi. alimentary canal ; umii. 

 ambulacral system or hydroccele ; dors, p, dorsal pore ; tnt. enteric sacs and coelome ; Inn-. 

 in", larval mouth ; Inrr. nrii. larval nrgan ; *'. oesophagus of adult ; '/. r. lobes of hydroccele ; 

 * /</. sejitum between the enteroccelic sacs. (A, after Ludwig ; B, from Ziegler's models.) 



in the form of an outgrowth, the hydroccele. The lower border 

 of this becomes five-lobed (Fig. 289, A], each of the lobes (r) 

 developing later into one of the radial ambulacral vessels ; the 

 central part nf the hydroccele subsequently forms a ring therm/?- 



