ZOOLOGY AND BOTANY. MICROSCOPY. ETC. 



65 



an outgrowth of the water-vascular ring with a tegumentary investment, 

 the group may be called that of the Angiochirota. In the second group — 

 that of the Echinoids — the region distinguished by the possession of 

 the primary tentacles has disappeared altogether, and it may, therefore, 

 be called that of the Achirota. In the third type, which is represented 

 by Asteroids, Ophiuroids, and Crinoids, we see important systems of 

 organs drawn into the tentacular region, which thereby gradually 

 acquires greater significance and independence ; this group it is proposed 

 to call that ol the Coelomachirota. 



The relations of these forms may be indicated by the following 

 diagram : — 



Ophiurqidea 



Aster oidea 



Crinoidea 



Echinoidea 



Holothurioidea 



Pent actffia 



I (Stem-form) 



The true homologies of the organs of different classes can only be 

 established by reference to the organization of the stem-form. Many of 

 what have been hitherto regarded as homologies are clearly analogies, 

 due to the fact that most of the structures compared are arranged in 

 fives, and to the inheritance from the stem-form of certain peculiarities, 

 such as the structure of the skeletal elements, and the tendency of the 

 mesenchym to form clefts. 



It cannot be doubted that the Echinoderms are derived from bilateral 

 creatures with an enteroccel ; it cannot be yet decided whether the 

 hydroccel is a derivate of a primitive excretory system of the bilateral 

 ancestors. There are good reasons for supposing that the conversion of 

 the bilateral into the radial structure was due to a fixed habit of 

 life. With regard to the corm-theory of the organization of certain 

 Asterids, it is urged that such colonies could not have arisen by budding, 

 but by certain organs (tentacles) acquiring greater independence. This 

 independence, which is to be regarded as a consequence of continued 



1889. r 



