•2'A Schuohert— Jackson on the Phytogeny of the Echini. 



on either side. That is, while an ambulacrum originates on 

 the ventral border of an ocular, each interambulacrum may be 

 considered as composed of two halves, the plates of which 

 originated on the left or right of the area in contact with the 

 adjacent oculars. If this is true, then the loss of an ocular 

 would cause a failure to develop of the plates that normally 

 went with it, also an abnormal position of an ocular should 

 cause an abnormal distribution of the associated coronal 

 plates' 1 (35, 36). 



The " variations from the pentamerons symmetry can all be 

 considered as monstrosities" (,50). 



Ambulacral areas. — The ambulacrum " is the most essen- 

 tial feature of a sea-urchin, and has a first importance in clas- 

 sification and morphology, on account of the varied structure 

 that it presents" (53). 



Inter amhulacral areas. — " The interambulacrum in Echini 

 functions chiefly as a space filler and a bearer of spines and 

 pedicellariae. The spines serve for protection and more or less 

 in locomotion, and pedicellariae as grasping, cleansing, and 

 protective organs. In spite of this secondary physiological 

 importance, the interambulacrum forms a large part of the test 

 of the sea-urchin in most types, and is of very great interest, 

 especially in Palaeozoic genera. The interambulacral plates 

 originate in direct contact with the ocular plates and quite 

 independently of the genitals. . . . 



" The full differential characters of the interambulacrum as 

 of the ambulacrum are expressed at the mid-zone of the adult. 

 Here are usually found the full number of columns of plates 

 characteristic of the species, also the typical tubercles, spines, 

 imbrication, or other characters which go to make up the 

 specific description. The ventral border in the basicoronal 

 zone represents the earliest formed plates and the youth of the 

 individual, as far as it can be gathered from the study of an 

 adult specimen, though the actually first formed plates may 

 have been resorbed in development. Passing dorsally, with 

 later added plates, new characters may come in until we get 

 the full differential features developed at or about the mid- 

 zone. Dorsal to the mid-zone we pass into the area of young 

 last formed plates which have not yet acquired the full char- 

 acters. Or again dorsally, we may find senescent features in 

 the loss of columns of plates. Passing from the basicoronal 

 row dorsally, we find in most Palaeozoic types, and many post- 

 Palaeozoic as well, stages in development strongly marked, 

 which stages can be correlated with the adult condition of 

 simpler genera or simpler species within the genus. The 

 interambulacrum in Echini has from one to fourteen vertical 

 columns of plates in each of the five areas, which represents 

 the least and greatest number known at present. There are 



