XVI ECHINODEEMATA 491 



hydrocoele on the left and of a vestigial right hydrocoele 



on the right; they are at first quite similar to one another. 

 Nevertheless the left side of the larva is clearly distinguishable 

 from the right because the pore -canal opens on the left side, and 

 the portion of the anterior coelom into which it opens becomes dis- 

 tended so as to form an ampulla. This ampulla is connected with the 

 rudiment of the left hydrocoele by a narrow neck which later forms 

 the stone-canal. On the right side the right hydrocoele remains 

 close to the right anterior coelom. Further, while the lumen of 

 the hydrocoele becomes enlarged, and the cells forming its wall 

 arrange themselves in a single layer of regular columnar epithelium, 

 the lumen of the right hydrocoele remains small and the cells remain 

 rounded and of irregular arrangement. 



BIPINNAEIA AND OPHIOPLUTEUS LARVAE COMPARED 



The larva has now reached the height of its development, and 

 from this period may be dated the beginning of its metamorphosis 

 into the adult Ophiuroid. Before considering this it will be instruc- 

 tive to compare the Ophiopluteus larva, point for point, with the 

 Bipinnaria at a similar stage of development, viz. just before the 

 appearance of the Brachiolarian arms. 



The two larvae agree in the following points : — 



(a) Both have a similar alimentary canal, consisting of a wide 

 shovel-shaped stomodaeum, cylindrical oesophagus (along the sides 

 of which and of the stomodaeum runs the V-shaped adoral ciliated 

 band), a globular stomach, and cylindrical intestine ending in ventral 

 arms. In both the alimentary canal is bent in a curve which is 

 concave ventrally. " 



(6) Both have a similar organ of locomotion, consisting of a 

 longitudinal ciliated band composed of two longitudinal sides and 

 anterior and posterior cross bars. In both this band is produced 

 into long arms or processes which extend its surface. 



(c) In both the space between gut and skin is filled by a watery 

 gelatinous connective tissue. 



(d) In both there are a pair of coelomic sacs which become 

 divided into anterior and posterior moieties ; and in both the rudi- 

 ment of the water-vascular system arises from the posterior end of 

 the left anterior sac, and into this sac the pore-canal opens. 



On the other hand the Bipinnaria and the Ophiopluteus larva 

 differ in the following points : — 



(a) The Bipinnaria has a long prae-oral lobe, and the loop of the 

 longitudinal ciliated band, which is bent along the ventral side of 

 this lobe, becomes separated from the rest as a distinct prae-oral band 

 of cilia. The prae-oral lobe is obsolete in the Ophiopluteus, and the 

 longitudinal ciliated band remains undivided. 



(b) The processes of the ciliated band remain soft in the 

 Bipinnaria, and are usually short in comparison with the size of 



