ANATOMY, HISTOLOGY, MORPHOLOGY, AND PHYLOGENY. Ecll. 15 
the mouth plates of the young Asterias , and it is concluded that there is 
no difference in the way the mouth parts of a typical representative of 
the group of starfishes known as Asterice ambulacrarice and Asterice 
adambulacrarice form in the development of the two groups, but 
that they are built on essentially the same plan, and the division of 
starfishes into' those with ambulacral and those with adambulacral 
mouths is not found in nature. The author supposes that the arm 
of a starfish is made up of somites or segments, and that, typically 
in Echinoderms, the calcification of each somite may be thought to sur- 
round the water-tube of the arm. This theoretical ring of calcification is 
most closely reproduced in its typical form in the plates surrounding the 
mouth, in both starfish and sea urchins. The ambulacrals and adam- 
bulacrals of a starfish are portions of the annular calcification of suc- 
cessive somites, and are serially homologous. In the ambulacrals the 
external part of the somite is aborted, while in the adambulacrals both 
the external and internal parts are missing, only the lateral portions 
remaining. In sea urchins the plates corresponding to the ambulacrals of 
a starfish are lost, appearing only round the mouth as the auriculae. On 
the other hand, plates corresponding to the adambulacrals of a starfish 
remain, and meet below the water- vascular system, to form the so-called 
ambulacrals of the sea urchiu. The marginals of starfish, which are 
most developed in pentagonal types, become the interambulacrals. 
Incidentally the author notices that Palmipes lays its eggs in long 
yellow strings (footnote, p. 102). The author passes on to discuss the 
primary spines of Echinoderms. In the young stages of many Echino- 
derms the animal is often provided with embryonic spines, relatively 
much larger, and of a different nature to those of the adult. Thus the 
young Asterias is provided with large spatulate spines, which the 
author compares to the modified fin-like spines of Ophiopteron , and which 
he believes may have been primitively swimming organs. There are two 
distinct kinds of spines in Asteroids, those which are simply outgrowths 
from a calcareous plate, and those produced from separate calcifications 
and afterwards articulated to a plate. To the latter category belong the 
embryonic spines of Asterias. Similar spatulate spines are found in the 
young Arbacia , which the author has frequently taken pelagic after 
absorption of the pluteus, though he has not actually seen it use its spines 
for swimming. He believes that this larval Arbacia represents an ances- 
tral form ( Archiarbacia ), from which Echinoids and starfish have sprung, 
with a disc-like body, and marginal flappers supported by a calcareous 
axis, the organism resembling an Ephyra , except for the calcification. 
In many young Ophiurids, on the other hand, the embryonic spines are 
hooked. These spines are permanent in the adult Ophiobrachion and 
Astrophyton. In Ophiurids also there are two kinds of spines, the 
ordinary straight spines of the adambulacral side plates, and the em- 
bryonic hooked spines, both kinds movable. It is left an open question 
whether these embryonic hooked spines are homologous with the spatu- 
late embryonic spines of Asterias. 
