ECHINODERMATA 437 



Ophiuroidea.— Although the larvce of the Ophiuroidea 

 and Asteroidea are so different in shape, their metamor- 

 phosis presents a certain resemblance. In the Pluteus larva 

 too the ambulacral and antambulaci^l surfaces are begun 

 separately, and only after their subsequent union give rise 

 to the complete star (JoH. Muller, Metschnikoff). In the 

 Pkiteus the five-rayed water-vascular rosette, which opens 

 to the outside world on the dorsal side of the larva, lies 

 ventrad of the oesophagus. It is on this that the first steps 

 in metamorphosis are manifested, for it is over its different 

 radii that the mesenchyma and contiguous larval skin be- 

 come thickened. In this way the fundament of the ambu- 

 lacral surface of the star is produced. Each of the five 

 radii of the rosette, which represent the future ambulacral 

 vessels, produces two lateral evaginations ; thus the larvlae 

 acquire the fundaments of the first feet, which are soon 

 followed by a second and a third pair, etc. While these 

 processes are taking place on the ventral side of the Pluteus, 

 the first indications of the antambulacral surface of the 

 Ophiuran have made their appearance on its dorsal surface, 

 in the form of five outgrowths of the larval skin. They are 

 arranged in a line, so that three of them lie on the larger 

 and two on the smaller part of the umbrella. The five 

 skeletal pieces arise in them as products of the mesenchyma 

 cells. Although the principal parts for the production of 

 the star are now present, nevertheless a total rearrange- 

 ment must take place to accomplish its formation. This 

 begins by the growth of the heretofore semicircular water- 

 vascular rosette, together with its appendages, around the 

 oesophagus, to form the water-vascular ring. With the 

 closure of the ring, the two vessels which at first were 

 farthest apart naturally have come to lie close together, and 

 at the same time the form of a star has now been reached, 

 first on the ambulacral surface. This is, however, not the 

 case on the antambulacral surface. Here also the dermal out- 

 growths (dorsal) undergo considerable changes in position ; 

 but it is not until the larval appendages begin to degenerate 

 that the antambulacral parts cover the ambulacral, and thus 

 complete the star. The internal parts of the larva — the 



