107 
[Fewkes. 
still adjoin the dorso-central, and the whole abactinal region of the 
body of the starfish is represented in Echinoids by that small area 
which lies within the circle of the oculars. In this area there are sev- 
eral small plates (periproctal plates, Sladen), which are simply ru- 
diments of the plates of the starfish in the same region. 
Of the plates of the arms we have the following calcifications of 
the starfish arm unrepresented in the sea-urchin. 
1. Dorsals. 
2. Connectives. 
3. Dorsolaterals. 
The bearing of the above comparisons and tlifcir theoretical im- 
portance are explained by the following supposition. It is supposed 
that the ray of the starfish is composed of a number of rings or 
joints strung together. These joints may be composed of two parts ; 
one enclosing the water vascular and nervous system, the other the 
digestive, forming the body cavity. The part related to the water 
vascular system forms the calcification of the actinal region of the 
arms. Each typical segment of the actinal region may be regarded 
as a calcareous ring formed of two halves, found one on each side of 
the medial line. These halves may or may not join. In the primary 
somites of the starfish the parts of the ring above (inside) the 
water system are developed and join. In the secondary segments, 
lateral parts are represented in the starfish. In the sea-urchin these 
latter form outside plates which join on the middle line. 1 
From this it appears that while the so-called adambulacrals of the 
starfish are homologous with the so-called ambulacrals of the urchin, 
the exact comparison should be with different regions of the ring 
in the two groups, for the ambulacrals of the starfish belong to the 
primary, those of the sea-urchin to the secondary segments. 
In the mouth plates we find the nearest approach to a complete- 
ring of calcification in both sea-urchin and starfishes, or we have, 
1 The term “primary segments” of the starfish arm may be used to apply to those 
segments, when the calcifications of the ideal ring inside the water system are devel- 
oped and those outside the same are wanting. 
■ The term “secondary segments,” on the other hand, refers to the adjoining somites, 
each of which lies between the primary in which neither the portion of the calcareous 
ring above the water tube nor that below it is calcified, but only those on either side. 
The primary and secondary somites are alternate segments of the arm. 
In the primary ambulacral the portion of the calcareous segment inside the water sys- 
tem is developed in the starfish and the outside portion is wholly wanting in the Echi- 
nus. In the secondary ambulacrals the lateral parts of the ring are represented in 
the starfish and the outside in the urchin. 
