IX 



PHYLUM ECHINODERMATA 



391 



tew. 



FIG. 316. Larva of Asterina, view 

 of the left side, showing the five- 

 lobed prominence (am&.) formed by 



the developing ambulacral system 

 on what is destined to become the 

 ventral surface of the body of the 

 Starfish ; tarv. org. larval organ. 



tertt 



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

 ing of the stomodeewm 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 ex- 

 terior both by mouth and anus : 

 soon, however, the larval anus be- 

 comes 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 ecto- 

 derm about the middle of the dorsal 

 surface, where an aperture is formed. 

 The pre-oral lobe appears at an 

 early stage as a dilatation at the 

 anterior end of the larva. This 



takes an antero-posterior direction, and assumes the character 

 of an elongated, almost cylindrical, hollow appendage at the 

 anterior end of the larva, consisting of a shorter, anterior, and a 



longer, posterior, part. 

 On the anterior sur- 

 face 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 ele- 

 vation, the rudiment 

 of a sucker by means 

 of which the larva be- 

 comes attached when 

 the metamorphosis is 

 about to begin. At 

 this stage the larva 

 (Fig. 316) is able to 

 creep by contractions 

 of the pre-oral lobe, 

 and also to swim 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- 

 coele, grows into the form of a horse-shoe with five lobes, each of 



FIG. 317. Asterina exigua. Young Starfish shortly 

 after the metamorphosis has b^en completed, viewed 

 from the oral side. circ. amb. circular ambulacral 

 vessel ; dors. p. dorsal pore and madreporic canal ; rad. 

 amb, radial ambulacral vessel ; st. stomach ; tent, tentacle ; 

 t. f. tube-feet. 



