164 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



Internal skeleton. 



In the crustaceans the cuticle in the region of certain mouth parts (as for 

 instance in tlic rofricn of the niandiblos) is folcU-d inward, forming chitinous "t<'n- 

 dons," or insertions for muscles, protecting the ventral nerve cord and v<'nous 

 blood sinus, and constituting the complex, apparently but not really, internal 

 endophragnnU skeleton of the thorax. It is a development of tliis endophragmal 

 skeleton of the crustaceans which forms the calcareous mouth plates ui the holo- 

 thurians, the complicated " Ai-istotle's lantern" of the echinoids, and, folded out- 

 ward bestead of inward, the long and complex arms of the crinoids. 



Skeleton of the heteroradiate echinoderms. 



Judging from the skeletal system the holothurians and echinoids are the most 

 primitive of the heteroradiate echinoderms. In both of these groups the longi- 

 tudinal axis of the digestive system passes (more or less ob-vdously) at right angles 

 thi-ough the center of the curler into which the longitudinal axis of the original meta- 

 mercs has become transformed, and in both there is present a coronal ring of 10 

 plates, 5 large and 5 smaller, the latter radial in position, this ring in the holothurians 

 being situated about the oesophagus at the opposite polo of the body from where it 

 is found in tlie echinoids. 



The bordering plates of each radial division always keep entirely distinct 

 from those of the adjacent series and never fuse with them, though they may com- 

 bine to a greater or lesser extent among themselves. The central series of plates 

 and the bordering plates in the urchins are typically subequal m size, though there 

 may be more or less difference; the individual plates of each series are always 

 similar and equal, or very nearly so. 



The embryology of the insects and crustaceans shows that development begins 

 at the anterior end of tlie bod_v, gradually extendmg itself posteriorly. Fusion 

 of segments and other simihir phenomena are firet e\'idencod in the anterior portion 

 of the larva, to which portion they are often confined. 



Thus the anterior situation of the calcareous ring of the holothurians would 

 suggest that in th<'se animals it is a new structure, just in the mcipient stage, this 

 hj^jothesis being strengthened by its somewhat indefinito character. 



Echinoids may be described as holothurians in which the ring of 10 plat(>s, 

 now of fixed and dcfmito size and interrelationships, has moved backward along 

 the body to the posterior end, so that it surrounds the anus instead of the mouth, 

 each plate leaving a trail of redupUcations of itself behind it to mark its passage. 

 In the echinoids the spicidated covering of the body as seen in the holothurians is 

 now reduced to a small circular area within the coronal ring, and even here the 

 spicules may be segregated into a single large plate. 



The traveling of the coronal ring in the echinoids from the original position 

 which it occupies in the holothurians to the opposite end of the body is clearly indi- 

 cated by the fact that new plates in the test are only formed between the ])lates of 

 the coronal ring and the plates already formed. In any series of imits addition to 

 the number occurs only at the free end, which is normally the place of increase. 



