Fewkes.] 
106 
[Dec. 5, 
these plates we find one and the same plates with the characters of 
both. Farther out on the arms this common calcification either ap- 
pears as an ambulacral rafter, or an interambulacral, and assumes 
very different forms ; but in the oral plates we have structures 
which partake of the character of both, or rather of our supposed 
calcareous ring surrounding the water-vessel. 
A comparison of the calcifications of starfishes and sea-urchins 1 
may be seen in the following table : — 
Starfish. 
Sea-urchin. 
1 . 
2 . 
3 . 
Ambulacyal rafters. 1. Wanting. 
Peripheral ambulacrals, generally ) ^ Ambulacrals 
called adambulaerals. ) 
Marginals. 3. Adambulaerals. 
The mouth parts of both starfishes and sea-urchins take the form 
of both ambulacral rafters and adambulaerals. 2 The condition of 
the mouth plates in the young starfish is similar to that of the sea- 
urchin for the ambulacrals of the starfish resemble in position the 
auricles of the sea-urchin. 
If now we compare calcareous plates of the body of the starfish 
and sea-urchin we find this interesting fact. All the plates of the 
abactinal region of the starfish are practically unrepresented in the 
sea-urchin. The interbrachials of starfishes are wanting in Echi- 
noids. 
The following may show in a tabular form the relation between 
certain 
plates in these groups : 
Starfishes. 
Sea-urchins. 
1 . 
Dorso-central. 
1. Dorso-central. 
2. 
Underbasals, radials. 
| Wanting. 
3. 
Connectives. 
4. 
Genitals. 
4. Genitals. 
5. 
Terminals. 
5. Oculars? 
6. 
Interbrachials. 
6. Wanting. 
The failure of the underbasals and radial body-plates to form in 
the sea-urchin has brought it about that the genitals and radials 
ir The homologies here presented are essentially the same as those already published 
by Ludwig as far as the relationship between the ambulacrals of the starfish and the 
adambulaerals of the sea-urchin is concerned. 
2 0r more accurately speaking we may say that the elements of an ideal calcareous 
ring surrounding the water vascular system is in the case of the mouth plates almost 
wholly consolidated into a ring. 
