514 
HUBERT LYMAN CLARK 
paractinopod holothurians, on the other hand, the development 
of the primary tentacles has been accelerated by their usefulness 
as tactile and locomotor organs, and, therefore, they have come 
to surpass the radial water vessels, not only in size but in their 
earlier and more rapid development. 
It would be easy to draw a picture of the hypothetical echino- 
holothurian ancestor in accordance with the theory here pro- 
posed, but it could hardly be worth while. Suffice it to point out 
that the five primary interradial tentacles of such an ancestor 
doubtless assisted in the pushing of food into the mouth, and be- 
cause of this use two very different lines of development opened 
up. Along one line, the tactile function was specialized and the 
organs became delicate, sensory tentacles, while along the other 
line, the mechanical function of scraping food, from the surface 
on which the animal crept, was specialized and the organs became 
hard, non-sensory teeth, accompanied by the necessary muscles 
and calcareous supports. The loss of a hard dermal skeleton in 
the one case, and its perfected development in the other, were 
naturally correlated accompaniments. In accordance with this 
theory, it may be possible to homologize the perignathic girdle 
of echini and the calcareous ring of holothurians. I am inclined 
to think, however, that such a homology does not exist. But 
it does seem to me probable that there is a true homology between 
the radially placed sphaeridia of echini and the paired positional 
organs, accompanying the radial nerves of synaptids. At present, 
however, I have no evidence to offer bearing on the point. 
SUMMARY 
1. Ostergren's view that the ancestral holothurian was a mud- 
loving Clasipod-like form is rejected for three reasons: (1) Too 
little weight is given to the difference between primary and sec- 
ondary simplicity of structure; (2) The ancestral holothurian was 
probably not a mud-loving but a plankton-feeding^ form; (3) 
By "plankton-feeding form," is meant an animal that lives on micro-organ- 
isms which it takes from the water about it. In this case, I would include also the 
gathering of micro-organisms from the rocks or hard bottom on which the 
ancestral form lived. 
