Fewkes.] 
108 
[Dec. 5, 
in other words, a segment formed of both the primary and the sec- 
ondary somites of the arm united. 
In conclusion it may be said that observations have shown that 
the oral plates of Asterina form like those of Asterias, and that the 
so-called adambulacral (secondary ambulacral) form between the 
ends of the ambulacral rafters (primary ambulacrals). From these 
observations we may conclude that the oral plates of Asterina, 
and Asterias, once supposed to be formed on an entirely different 
plan, so different in fact that this supposed difference has been 
made one basis of classification, resemble each other in an embryo- 
logical point of view, and best illustrate our hypothetical calcareous 
ring. 
Summary. 
1 . There is no difference in the way the mouth parts of a typical 
representative of the group of starfishes known as Asterise Ambu- 
lacrarise and Asteriae Adambulacrarise form in the development of 
the two groups. 
2. The arm of a starfish is made up of somites or segments, and 
the water vascular system of vessels may be supposed to be prima- 
rily surrounded by a calcification. The theoretical ring of calci- 
fication is most closely reproduced in its typical form in the plates 
surrounding the mouth. 
3. Ambulacrals and adambulacrals are portions of the annular 
calcification of successive segments and are serially homologous. 
4. The ambulacrals of starfishes are not represented in sea- 
urchins except about the mouth where they appear as auricles. 
5. The adambulacrals of starfishes represent the ambulacrals of 
sea-urchins, and complete the external portion of the problemati- 
cal ring of calcification which is absent in Asteroids. 
6. The marginals of Asterias are homologous to the so-called 
adambulacrals of sea-urchins. 
Primary Spines of Echinoderms. 
Anyone who studies the young of our common starfish, Asterias, 
will be struck by the relatively large size of those structures called 
spines which are appendages of the early formed calcareous plates. 
These spines are not only relatively much larger than those later 
formed but also differ somewhat from them in shape. They begin to 
form before the larval starfish passes out of that condition in its 
