36 



BULLETIN 



UNITED STATES NATIONAL MUSEUM. 



oidea), all of which appear from underneath the ventral edges of 

 the oculars, each plate in turn being pushed orally 1 . 



In any event, whatever the pre-Hudsonaster evolution was, we 

 are on safe ground in indicating what the developmental changes 

 were, starting with this genus. We will not take up all of the skele- 

 tal parts and discuss them in detail, but will attempt only the more 

 essential portions. 



Ontogeny. Ontogenetic stages of development in the ossicles 

 of asterids can readily be made out to some extent in any well pre- 

 served specimen. The variations can best be seen in those forms 

 with the least number of plates and therefore in the more primitive 

 genera. Here all of the primary ossicles are introduced at the tips 

 of the rays in the seven primary columns. The youngest pieces are 

 also the smallest and the simplest. At first they are globular, 



FIGS. 4 AND 5. DORSAL AND VENTRAL VIEWS OF THEORETIC TYPEMBRYO OF STELLEROIDEA. Ax, VEN- 

 TRAL OR MARGINAL AXILLARIES OR BASAL INFRAMARGINALS; CD, CENTRO-DORSALJ R, PRIMORDIAL 

 RADIALSJ Sml, DORSAL INTERRADIALS OR PRIMORDIAL SUPRAMARGINALS. 



smooth, and loosely adjoining. Tracing them down the columns, 

 the specific characters are seen to develop and finally the basal pieces 

 are found to be the most modified of all. This is particularly true 

 of the dorsal columns where the most ornate plates are the oldest. 

 In the different species of Hudsonaster one can also trace the changes 

 chronogenetically, as for instance in the basal radialia and supra- 

 marginalia. In the oldest species these plates are not at all, or but 

 very slightly stellate, more so in H. incomptus, and most so in the 

 youngest and largest species, H. rugosus. 



In the stocks with more complex skeletons, the ontogenetic varia- 

 tions in the plates can be traced in the same way. However, as these 

 forms almost always have more or less of accessory plates that are 

 introduced nearly throughout the entire skeleton during the juvenile 

 growth and even at maturity, the study is complicated by the inter- 

 mixture of ossicles of varying age. 



i See Jackson, Phytogeny of the Echini, Mem. Boston Soc. Nat. Hist., vol. 7, 1912. 



