XVI 



ECHINODEEMATA 



469 



posterior coelom becomes entirely cut off from the anterior coelom • 

 It IS termed the epigastric coelom by Goto, who thought that it 

 was formed by the growth of an independent longitudinal septum 

 but this error has been corrected by Gemmill. 



By the time that these changes have been accomplished the 

 brachiolar arms have been formed ; and in the centre of the circle 

 formed by them a circular disc of thickened glandular ectoderm 

 appears. This is the organ for permanent fixation {fix, Fig. 362) 

 Holdmg on by its brachiolar arms the larva brings this disc into 

 close contact with the substratum and thus •permanently fixes itself. 



The larva may now be said to be 

 differentiated into a posterior region 

 containing the stomach, the intestine, 

 the hydrocoele, and the right and left 

 posterior coeloms; and into an anterior 

 region, consisting of the prae-oral lobe, 

 containing the mouth, oesophagus, and 

 the anterior coelom. The anterior region 

 may be termed the stalk, the more 

 posterior the disc. Once the larva has 

 become firmly attached the stalk is pro- 

 gressively shortened (Fig. 362, B). 



The stone-canal makes its appear- 

 ance as an open groove of ciliated 

 epithelium, situated on the anterior 

 aspect of the septum dividing anterior 

 from left posterior coeloms. It begins 

 just beneath the inner end of the pore- 

 canal, and it runs down to the spot 

 where the anterior coelom is beginning 

 to be pinched from the hydrocoele. 



On the posterior aspect of the septum 

 there appears a groove-like outgrowth of 

 the left posterior coelom. This groove 

 is the rudiment of the peri-oral coelom. It grows into a tube which 

 extends in the form of a slight crescent beneath a faint bulge of the 

 stomach, which is the rudiment of the adult stomach. 



The stomodaeum becomes disconnected from the endodermal 

 portion of the oesophagus : it persists for a brief time as a shallow 

 pit of the ectoderm, but eventually disappears entirely, and the 

 oesophagus becomes, as metamorphosis proceeds, a less and less 

 conspicuous appendage of the stomach. The anus of the larva is 

 also obliterated, and the intestine becomes shortened till it forms a 

 very short tag attached to the stomach. According to Gemmill this 

 tag persists throughout metamorphosis, and from it the rectum of 

 the adult is developed. 



Five thickened lobes now appear on the ectoderm covering the 

 right posterior coelom. These are the first traces of the arms of the 



Fig. 361. — Ventral view of a 

 Bipiunaria of Aslerias vulgaris 

 of the same age as that shown 

 in Pig. 360, in order to show 

 the mutual relations of the 

 coelomic cavities. (After Goto.) 



Letters as in preceding figure. 

 In addition, X^p^c^, right ventral liorn 

 of left posterior coelom ; r.p.c, right 

 posterior coelom. 



