878 Agassiz—Paleontological und Hmbryological Development. 
young stages of this group of i eg ap not one of these 
structural features is as yet developed. e actinostome is 
simple, the poriferous zone has the same simple structure from 
the actinostome to the apex, the primary tubercles are large, 
few in number, surrounded by spines which would more readily 
pass as the spines of Cidaridze than of Spatangoids. The fas- 
cioles are either very indistinctly indicated, or else the special 
ines have not as yet made their appearance ; the ambulacral 
suckers of the anterior zones are as large and prominent as 
those of the young stages of any of the regular Hchini. It is 
only little by little, with advancing age, that we begin to see 
signs of the specialization of the anterior and posterior parts of 
the test, that we find the characteristic anal or lateral soma 
making their appearance, only with increasing size that t 
spines lose their Cidaris-like appearance, that the petals basil 
to be formed, and that the simple cree develops a prom- 
inent posterior lip. In the genus Hemiaster, the young stages 
are specially interesting, as long before the appearance of the 
petals, while the poriferous zone is still simple, the total sepa- 
ration of the bivium and of the trivium of the ambulacral sys- 
tem, so characteristic of ithe earliest Spatangoids (the Dysas- 
teridw), is very apparent. 
rom this rapid sketch a the changes of growth in the prin- 
cipal families of the recent Echini we can now indicate the 
transformation of a more general character through which the 
groups as a whole pas 
In the first place, While still in the Pluteus all the young 
Kehini are remarkable for the small number of coronal plates, 
and for the absence of any sae between the actinal and 
 abactinal systems and the test proper. They all further agree 
in the large size of the primary spines of the test, whether it be 
the young s a Cidaris, an Arbacia, an Echinus, a Clypeaster, 
or a Spatangoid, They all in their youngest stages have 
simple ated ambulacral zones; beyond this, we find as 
changes characteristic of some of the Desmosticha, the special- 
ization of the actinal system from the coronal plates, the forma- 
tion of an anal system, the rapid increase in the number of the 
coronal plates, with a corresponding increase in the number of 
the spines and a proportional reduction of their size, the form- 
ation of an abactinal ring, and the change of the simple verti- 
cal poriferous zone into one composed of independent ares. 
In the Spatangoids and Clypeastroids we find common to 
both groups the shifting of the anal system to its definite place, 
the modifications of the abactinal part of the simple ambulacral 
* For this sketch of the embryology, of the Petalosticha I have examined th 
young of Kchinolampas, a Kehinocardium, Brissopsis, Agassizia, Spe- 
tangus, Brissus, and Hem 
