216 Mr MacBride, Variations in Asterina gibbosa. [May 14, 



processes. (1) The dififerentiation of the body into a disc and a 

 stalk, and the absorption of the latter, (2) The growth of the left 

 hydrocoele till its two ends meet and it forms a ring. (3) The 

 similar growth of the left posterior coelom to form a ring, this giving 

 off at the same time five diverticula, which form the future arms 

 and become applied to the primary lobes of the hydrocoele which 

 form the radial canals of the water-vascular system. Now it will 

 be easily seen that process (2), and with it the whole metamor- 

 phoses, can be held in check by the right hydrocoele developing 

 more than usual. In the very remarkable specimen which I now 

 describe the process (1) is almost complete, so that I took the 

 animal for a starfish, the metamorphosis of which was concluded. 

 Sections however revealed the fact that the ends of the left 

 hydrocoele had not met, but were in fact prevented from doing so 

 by the right hydrocoele, which was large, with three distinct lobes. 

 There was a single pore-canal, but two stone-canals, one opening 

 into each hydrocoele, and meeting above close to the inner end of 

 the pore. Another specimen showed just the reverse of this, viz. : 

 two pore canals on opposite sides of the body, joining and opening 

 by a common pore close to the inner end of the single stone- 

 canal opening into the left hydrocoele. In another case the 

 metamorphosis was complete, and the right hydrocoele was a thin 

 walled sac. A second short stone-canal connected it with the 

 anterior coelom, and a second pore-canal also opened into the 

 latter, but united with the normal pore-canal to open by a single 

 pore. 



In a short communication to the Royal Society I compared the 

 coeloraic cavities of the larva of Asterina to those of the Balano- 

 glossus larva. On this view, the madreporic pore would correspond 

 to the proboscis pore, and the collar pores would be unrepresented, 

 as of course neither hydrocoele has a direct communication with 

 the exterior. In one larva, however, to my great surprise I found 

 such a pore co-existing with the ordinary madreporic pore. It 

 opened between the third and fourth lobes of the hydrocoele. 



Finally, a larva should be mentioned in which the only 

 hydrocoele present was small and two-lobed, communicated by 

 only a slit with the coelom, and was placed far in front of the 

 stomach. 



In conclusion, I think one may fairly say that these variations 

 prove at any rate beyond all doubt that the sac I have called 

 the right hydrocoele is a structure of the same nature as the left 

 hydrocoele. Since the larva is in its early stages quite bilaterally 

 symmetrical, we may assume that the free-swimming pelagic 

 ancestor of the Echinoderms had two equally developed hydro- 

 coeles ; and hence the variations I have described come in the 

 majority of cases under the head of atavism, using the word 



