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 the region of the mandibles) is folded inward, forming chitinous "ten- 

 dons," or insertions for muscles, protecting the ventral nerve cord and venous 

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

 endophragmal skeleton of the thorax. It is a development of this endophragmal 

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

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

 ward instead 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 obviously) at right angles 

 through the center of the circle into which the longitudinal axis of the original meta- 

 meres 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 ossophagus at the opposite pole of the body from where it 

 is found in the 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 in 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 the body, gradually extending itself posteriorly. Fusion 

 of segments and other similar phenomena are first evidenced 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 these animals it is a new structure, just in the incipient stage, this 

 hypothesis being strengthened by its somewhat indefinite character. 



Echinoids may be described as holothurians in which the ring of 10 plates, 

 now of fixed and definite 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 reduplications of itself behind it to mark its passage. 

 In the echinoids the spiculated 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 hi the test are only formed between the plates of 

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

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



