152 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



Relationship between the digestive tube and asymmetry. 



In all the echinoderm classes it is the digestive tube that controls any departure 

 from the primitive radial symmetry. In the two groups in which the digestive 

 tube itself is radially symmetrical, with its axis always at right angles to the plane 

 of the circle representing the somatic axis (the Asteroidea and the Ophiuroidea), 

 none but the most trifling departures from the radial symmetry occur; but in the 

 other three groups (Pelmatozoa, Echinoidea and Holothuroidea) in which the diges- 

 tive tube retains its original character, its anteroposterior axis often becomes inclined 

 to the plane of the circle representing the somatic axis, or, by a migration usually 

 of the anus, sometimes of the mouth, occasionally of both, becomes modified into a 

 crescent or horseshoe-like curve, in which event the animal immediately develops a 

 bilateral symmetry which is accentuated roughly in proportion to the departure 

 of this axis from its normal position, though decreasing again if the anus approaches 

 close to the mouth. 



The axis of the digestive tube always maintains its character as a true axis, 

 and is continually endeavoring to assert itself and to overcome the conservatism or 

 inertia of the circular somatic axis, and to impose its ancestral bilateralism upon a 

 normally radial body. In this it has been to a large degree successful among the 

 more specialized types, in the so-called irregular urchins and in many of the holo- 

 thurian groups, which have secondarily assumed a bilateralism which, in view of 

 the limitations imposed by the primarily radial structure of the animals, may be 

 regarded as extreme. The elongation of the body among the holothurians I regard 

 as due to the dominance of this axis over the somatic, and not in any way suggesting 

 wormlike affinities. 



Many of the crinoids advanced far along similar lines ; but the shrinking of the 

 calyx as well as the close approach of the two ends of the digestive tube and the 

 consequent neutralization of the bilateral tendency have combined to inhibit its 

 effect, especially in the later forms. 



In the crinoids the anus opens in the interambulacral area of the disk opposite 

 the anterior ray (figs. 20, p. 69, and 117, p. 183). It is not simply an opening in 

 the integument, but is situated usually at the summit., more rarely on the side or 

 at the base, of a conical proboscis, which may be expanded into a huge sac, and is 

 always large. 



In the species of the family Comasteridae the interambulacral area including 

 the anal proboscis is typically greatly enlarged, occupying nearly the entire surface 

 of the disk (figs. 25-28, p. 69). The digestive tube makes about four complete 

 concentric coils, all centering directly beneath the anal proboscis (fig. 21, p. 69). 

 The digestive tube turns to the right, so that the coils are wound in the direction 

 taken by the hands of a clock. 



Additional growth by a digestive tube of the type occurring in the species of 

 Comasteridae, or dilation due to gorging with food, tends to broaden the various 

 coils, and also tends to force the mouth toward the right; because of the small size 

 of the body cavity, the chief effect is evident hi the latter direction. Thus it is that 

 in many of the species of Comasteridse we find the mouth pushed from its normal 



