IX 



PHYLUM ECHINODERMAtA 



383 



the left a five-lobed outgrowth, the hydrocoele, which forms the 

 foundation of the entire ambulacral system of the adult : a right 

 hydrocoele is only represented by a small vesicle which in normal 

 embryos undergoes no further development. Before the hydro- 

 coele is developed and before the right and left ccelomic 

 pouches have become cut off, two apertures make their appearance 

 on the surface of the larva : one, on the ventral side, is the open- 

 ing of the stomodceum or larval mouth ; the other, on the dorsal 

 side, is the dorsal pore. The mouth subsequently opens into the 

 larval stomach, and for a time the enteric canal of the larva opens 

 on the exterior both by mouth and anus : soon, however, the 

 larval anus becomes closed up. The dorsal pore is developed as an 

 outgrowth of the anterior part of the enterocoele, a little to the left 

 of the middle line, meeting a thickening of the ectoderm about the 

 middle of the dorsal surface, where an aperture is formed. 



B iarv. org 



fas i'. ma 



ec 



FIG. 320. Larva of Asterina gibbasi. A. diagrammatic lateral view; the alimentary 

 canal dotted, the ambulacral system striated, the ectoderm shaded. B, Larva seen from 

 the left as an opaque object, the body-wall of the left side removed ; hydrocoele separated 

 off from left enteric sac and partly surrounding oesophagus, ali. alimentary canal ; amb. 

 ambulacral system or hydrocoole ; dors. p. dorsal pore ; ent. enteric sacs and coelome ; 

 lure, mo. larval mouth ; laro. or&r.. pre-oral lobe ; ces. oesophagus of adult; r, r. lobes of 

 hydrocoele ; sept, septum between the enterocoslic sacs. (A, after Lndwig ; B, from 

 Ziegler's models.) 



The pre-oral lobe appears at an early stage as a dilatation at the 

 anterior end of the larva. This takes a dorso-ventral direction, 

 and assumes the character of an elongated, almost cylindrical, 

 hollow appendage at the anterior end of the larva, consisting of a 

 shorter ventral, and a longer dorsal, part. On the anterior surface 

 of the pre-oral lobe a flattened area appears surrounded by a raised 

 rim, which is beset with specially large cilia : this is the larval 

 organ. In the middle of the larval organ appears an elevation, 

 the rudiment of a sucker by means of which the larva becomes 

 attached when the metamorphosis is about to begin. At this 

 stage the larva (Fig. 321) is able to creep by contractions of the 

 pre-oral lobe, and also by the action of the cilia, more especially the 

 cilia of the larval organ. 



The hydrocoele, at first a five-lobed outgrowth of the entero- 



