THE STAEFISH SOLASTEE ENDECA. 51 



that, while in point of external form the left side of the larva becomes the oral surface 

 of the Starfish and the right side the aboral, in reality the epiderm of the oral surface 

 is chiefly derived from that of the anterior part of the larva, and, conversely, the 

 epiderm of the aboral surface originates chiefly from that of the posterior part of 

 the larva. 



Five hydrocoele-lobes appearing to begin with, the rest being added later in such a 

 way that the whole series may properly be described as beginning in the hydroporic 

 region and progressing thence in the watch-hand direction as seen from the oral 

 surface. 



Aboral arm-rudiments also incomplete in number for a time. 



Disappearance of the preoral lobe ; atrophy of the sucker as the tube-feet become 

 functional. Development of mouth and anus ; outgrowth of the arms and appearance 

 of the rosettes of spines. 



(B) Internally. 



1. In Larva. — Archenteron divides into anterior and posterior coeloms and a middle 

 chamber, the enteron or gut. 



Anterior coelom provides cavity of preoral lobe and a central part which gives rise to 

 right and left lateral diverticula, as also to dorsal sac, hydroporic canal, and perihsemal 

 pouch IX-I. Bottom of left lateral diverticulum becomes hydrocoele groove and gives 

 rise to radial pouches ; five of the latter appear almost simultaneously, and are to be 

 numbered in the manner indicated for the corresponding external swellings ; groove of 

 stone-canal opens into hydrocoele groove between origins of pouches I and 11; pouches VI 

 and VII are added ; hydrocoele groove and stone-canal groove begin to be roofed over. 



Posterior coelom sends out dorsal and ventral horns in sagittal plane, and gives 

 rise to a series of six perihsemal pouches and to diverticula for the pharyngeal 

 coelom. It also tends to extend over the left side of the enteron, and in a less degree 

 over the right. 



2. In Metamorphosis. — Preoral coelom disappears ; central part of anterior coelom 

 becomes axial sinus ; right lateral diverticulum gives rise to epigastric coelom ; 

 hydrocoele groove is separated off from left lateral diverticulum and extends to form 

 ring-canal, the last two radial pouches being now added. Rest of left lateral diverti- 

 culum, along with part of axial coelom, forms the internal oral circular sinus. 



Posterior coelom with its two horns and two pharyngeal diverticula enlarges to form 

 hypogastric and peripharyngeal coeloms. The last two perihsemal pouches are given 

 off, and along with the others form the external oral circular sinus. Tip of dorsal horn 

 gives rise to pocket with special cells, the rudiments respectively of the aboral circular 

 sinus and the genital rachis. 



Enteron expands greatly and shows blunt swellings in each ray. Mouth and anus 

 form, as well as the paired radial and the rectal caeca. The anus is in interradius V/VI. 



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