MUSEUM OF COMPARATIVE ZOOLOGY. 43 



Astropbytidte. His reasoning seems to me conclusive, and I do not 

 regard the madreporite as a fixed point of reference in Echinoderm 

 morphology. 



2'ermiiials. — The terminals in Amphiura and Asterias have so many- 

 points in common that I do not hesitate to regard them as morphologi- 

 cally identical. That they do not appear at the same time as compared 

 with the other primary plates does not appear of importance enough to 

 destroy the argument for their identity built upon their many resem- 

 blances of mode of growth, position, and relation to the median water 

 tube of the ray. 



Sladen considers that the occurrence of plates which he calls " under- 

 basals" in the Asterid nullifies Studer's argument that the arrangement 

 of primitive plates in Asterids corresponds with the monocyclic Crinoids. 

 I do not subscribe to Studer's idea, but on the other hand I cannot but 

 ask if Sladen has not overestimated the morphological value of his sup- 

 posed discovery of the " under-basals " of Asterids in this connection. 

 It may be well to remember that in Asterias the plates which Sladen 

 likens to the Crinoid " under-basals " do not appear until after at least 

 the Asterias may have had a monocyclic stage. At tlie time the " un- 

 der-basals form," we might suppose that the starfish had passed out of, 

 or become more developed than, the " monocyclic stage." 



From their place of origin and their subsequent growth the terminals 

 of Asterias and Amphiura are probably homologous. Moreover, it is 

 probable that these plates are not to be compai-ed with the other plates 

 of the arm, which originate between them and the axis of the body. It 

 had seemed to me possible to find a serial homology between this plate 

 and those of the arm joints ; to compare it, for instance, with a consoli- 

 dated dorsal, two laterals,* and possibly a ventral ; to find, in other 

 words, that the portion of the ray in which they lie is a true arm divis- 

 ion or "joint." My attempts, however, have not been rewarded with 

 great success, and it seems more probable that they are not comparable 

 with other arm plates. Students of Crinoid morphology do not find the 

 homologues of terminals in this group, and there is evidence that they 



* Sladen says (op. cit., p. oO) : " The comparatively large size of the terminal 

 plate at an early stage of the young Asterid is due, in my opinion, to a coalescence 

 of primitive lateral plates with the primitive, or first formed, rudiment of the ter- 

 minal, — a circumstance wliich further strengthens my view of the secondary 

 character of the terminal plate." It would, however, seem from the growth of tlie 

 plate that such a coalescence does not take place, or at ail events I was unable to 

 observe it. It is suggested that the absence of lateral plates (marginals 1) allows 

 the sides of the terminals to grow into their places. 



