474 



INVERTEBRATA 



CHAP. 



this is very necessary. Similar holes are formed in the transverse 

 septa in the Bipinnaria larva, as we have already seen. 



The hydrocoele arises as a bulge on the left side of the posterior 

 part of the anterior coelom, whilst the madreporic vesicle is formed 

 as a bud from the posterior end of the anterior coelom, a little to 

 the right of the median line. It becomes hollowed out, and is for a 

 time attached to the wall of the anterior coelom by a string of cells, 

 but this is soon broken and the vesicle detached. A right hydrocoele 

 with five well-developed lobes, or sometimes with only three or only one 

 lobe, is sometimes developed, and often in this case the madreporic 

 vesicle is suppressed, which is the reason why we formerly regarded 

 the two structures as homologous. Even before metamorphosis 

 begins the left posterior coelom is wider than the right, and begins to 

 send out a ventral horn which underlies the right coelom, and the 

 peri-oral coelom originates as a pocket of the left posterior coelom. 



FIG. 364. Views of a free-swimming larva of AstcriiKi ijililmm five or six days old. 



(After Ludwig.) 

 A, from in front. B, from the side and above, ji.r, lixing disc. prJ, prae-oral lobe. 



In Asterina gibbosa, as soon as metamorphosis commences the 

 stone-canal makes its appearance as an open groove on the anterior 

 face of the transverse septum, as in Asterias. This groove becomes 

 closed in the middle, but opens at one end into the hydrocoele, and at 

 the other end into the anterior coelom just below the opening of the 

 pore-canal. Then five outgrowths of the coelom shaped like inverted 

 wedges are formed. Of these outgrowths, one arises from the anterior 

 coelom and four from the left posterior coelom (p.h, Fig. 366). They 

 project against the ectoderm and alternate with the five lobes 

 of the hydrocoele. These outgrowths are soon cut off from the 

 coelom, and lie between ectoderm and coelomic wall as flattened 

 vesicles. They are the rudiments of the perihaemal system of 

 spaces. 



The arms grow out as blunt outgrowths from the region of the 

 body occupied by the left posterior coelom, and into each of them an 



