BASAL PLATES IN CRINOIDEA CAMERATA 545 



characters; but we have no evidence of such a reversion, and, until 

 such evidence is brought forward, theories of descent demanding 

 the reappearance of lost sutures should be carefully scrutinized. 



DEVELOPMENT OF THE INTESTINE AND THE CONSEQUENT 

 ZONES OF POTENTIAL WIDENING 



"In all the Echinoderm classes it is the digestive tube that 

 controls any departure from pentamerous radial symmetry." 

 This statement by Clark 1 may perhaps be too sweeping in extent, 

 especially when we consider the potency of atrophy and compen- 

 sating hypertrophy (p. 533) in distorting symmetry; but the fact 

 that the digestive tube is one of the most powerful factors in 

 distorting symmetry cannot be too strongly emphasized. The 

 most striking and therefore the most widely known effect of this 

 power is that shown in the various distortions of the mouth and 

 ambulacral grooves by excessive growth of the hind-gut. These 

 tegminal distortions have been so frequently described that there 

 is no necessity of reviewing them here; distortions produced by 

 the intestine in the basal and radial cycles have, however, received 

 too little attention. 



In the development of the digestive system in Antedon the 

 gastric sack is elongated horizontally into a form somewhat resem- 

 bling the human stomach, having a large end into which the funnel- 

 shaped oesophagus opens, and a small end with a caecal termination, 

 which is the potential intestine. Upon further development the 

 intestine is also horizontally prolonged, and coils to the right, 

 around the stomach, in the space left for it by the enlargement of 

 the calyx; 2 before the coil is completed, however, the anal appears 

 and the intestine directs itself toward that plate. The pressure 

 exerted by the intestine upon the anal tends to keep separated the 

 posterior radials and prevents the right-posterior radial from 

 encroaching upon the anal. 3 Soon, however, the intestine turns 

 upward and carries the anal with it into the tegmen. The 

 thrust exerted by the outward growth of the intestine which dis- 

 places the anal and also the radianal (see p. 538) must not be con- 



1 Ref. 16, p. 152. 



2 Ref. 12, pp. 227-28. 3 Radianal. See Ref. 16, p. 333. 



