44 



INTRODUCTION TO CLASSIFICATION. 



continuous skeleton, but those of Opliiuridea and Echinidea 

 possess a very remarkable bilaterally symmetrical, continuous, 

 calcareous skeleton, which extends into, and supports the pro- 

 cesses of the body (Fig. 21). 



A distinctly defined alimentary canal early makes its ap- 

 pearance in these Echinoderm larvae. It is divided into a well- 

 marked oral and oesophageal portion, a globular stomach, and a 

 short intestine terminating in an anal aperture (Figs. 18 and 19). 

 All the parts of the alimentary canal are disposed in a longi- 

 tudinal and vertical plane, dividing the larval body into two 

 symmetrical halves ; but the oesophageal and intestinal portions 

 are so disposed as to make an angle, open towards the ventral 

 side, with one another. No nervous, or other organs, besides 

 those indicated, have as yet been discovered in these larvae. 



Fig. 19. 



Fig. 19. A young Asterid larva (after Mttller). A. Ventral. B. Lateral view of the 

 larva. C. Commencing rudiment of the starfish, a. Mouth, b. (Esophagus, 

 c. Stomach, c'. Intestine, o. Anus. x. Anterior, and y, principal ciliated band. 

 h. Ca-cal diverticulum, forming the rudiment of the ambulacral vascular system, and 

 opening externally by the pore, g. k'. Perisoma of the starfish. 



Fig. 20. 



-5S3ES*" ^f/~' t ^^'-1 



Fig. 20. Development of a Holotkurid (after Miiller). A. Early condition of larva 

 B B, C. Later stages, f, g, h, the ambulacral vascular system. 



