1894.] Larva of Asterina gibbosa. 215 



the prgeoral lobe, and a right and left posterior portion ; these 

 latter being separated by the gut and the dorsal and ventral 

 mesenteries, in which the alimentary canal is suspended. The 

 anterior coelom is prolonged right and left into two diverticula 

 which overlap the right and left posterior coeloms respectively. 

 Of these the left has already become five-lobed, and is the 

 rudiment of the water-vascular ring and radial canals in the adult. 

 It opens by a slightly constricted neck into the anterior ccelom, 

 and a ciliated groove which has been constricted off from this 

 neck, is the future stone-canal. The right diverticulum is, as we 

 shall see, a rudimentary fellow to the water-vascular system. Its 

 walls are continuous with those of the anterior coelom, but its 

 cavity, though occasionally opening into the anterior coelom by a 

 very fine slit, is usually completely closed. It persists throughout 

 life as a completely closed sac, embedded in the madreporic plate. 

 The stone-canal does not open to the exterior ; it only leads from 

 the water-vascular rudiment, or " hydrocoele," into the anterior 

 coelom ; the latter, however, opens to the exterior through a short 

 ciliated " pore-canal," the outer aperture of which, situated at first 

 to the left of the mid-dorsal line, is as the larva grows displaced 

 to the right. This pore-canal is the first of the numerous canals 

 which traverse the madreporite of the adult, and the anterior 

 coelom persists in part as the axial sinus of the adult. The 

 variations which I wish to describe concern chiefly the right 

 " hydrocoele." Its normal appearance is that of a completely 

 closed sac lined by a flattened epithelium. In two specimens I 

 have found it two-lobed, these lobes having the shape and 

 cylindrical epithelium characteristic of the lobes of the left hydro- 

 coele. In both cases it opens by a narrow slit into the anterior 

 enterocoele, and the stone-canal and madreporic pore are normal. 



In another specimen it is large and five-lobed, better developed 

 in every way than its left fellow, differing only in its slightly 

 more dorsal position, and its narrower communication with the 

 anterior enterocoele. In this specimen, owing no doubt to the 

 development of the organs of the right side being equal that on 

 the left, the pore is median. 



In another and very remarkable specimen the right hydrocoele 

 was represented by two rudiments, one more dorsally situated and 

 one more ventral. The more dorsal rudiment has the ordinary 

 appearance, and it opens by a slit into the anterior coelom. This 

 latter possesses close to this opening a second pore- canal, much 

 smaller than the first or normal pore-canal, which is in its normal 

 position. The more ventral rudiment consists of two lobes having 

 the distinct hydrocoele structure and opening into the anterior 

 coelom also. 



The metamorphosis of the larva is brought about by three 



