200 ECHINODERMATA—ASTEROZOA SUB-BRANCH II 
The ventral position of the madreporite in Palaeozoic Star-fishes and the 
absence of mouth shields in Palaeozoic Ophiuroids point to an intimate 
relationship between the two groups; and this inference is still further con- 
firmed by our knowledge of several recent and fossil intermediate forms 
(Astrophiura, Protaster, Brisinga, ete.) 
If one places a Star-fish or Brittle-star with the mouth uppermost, it will 
be seen that the actinal side corresponds with the tegmen, and the central 
disk with the base of a Pelmatozoan. When oriented in this manner, the 
position of the principal organs (ambulacral, circulatory, and nervous systems) 
is the same in both groups. The homology between the arms of an Astero- 
zoan and those of a Crinoid or Cystid, or the ambulacral fields of a Blastoid, 
can also hardly be doubted. But efforts to interpret a homology between 
plates of the dermal skeleton as developed in either group have been only 
partially successful; the reason being that these structures became variously 
modified and specialised throughout the different classes at an extremely 
early period. 
A comparison of the ontogenetic stages passed through by the Pelmatozoa 
and Asterozoa reveals nothing definite in regard to their close relationship ; but 
the identity in position of the principal viscera argues strongly in favour of their 
common origin. The Asterozoans are most nearly comparable with certain of 
the Cystideans (Agelacrinus and the Callocystidac). But that they are the 
direct descendants of the Cystoidea appears very improbable, for both geological 
and morphological reasons. The fact is, that both types appear simultaneously 
and in a high state of development, each being totally distinct from the 
other, as far back as the Cambrian. 
Class 1. OPHIUROIDEA. Gray. Brittle-stars.! 
Asterozoans having a more or less sharply defined central disk containing a 
simple digestive cavity which does not radiate into the slender rounded arms, and 
has no anal opening. Reproductive organs confined to the disk. Arms with an 
axis composed of jointed calcareous disks, encased with plates or covered with a 
leathery skin, and destitute of open ambulacral grooves. Madreporite constantly on 
the actinal (oral) side of the disk. 
Ophiuroids are distinguished from the typical Star-fishes by their cylin- 
drical flexible arms, which are sharply separated from the central disk, and 
do not contain diverticula of the alimentary canal nor of the sexual organs. 
1 Literature : 
Liitken, C. F., Additamenta ad historiam Ophiuridarum (Kongl. dan. Vidensk. Selskabs Skrifter, V. 
and VIII.), 1858-69. 
Lyman, T., Ophiuridae and Astrophytidae (Ilustr. Cat. Museum Comp. Zoology, Cambridge, Nos. 
I.-III.), 1865. 
Lyman, T., Ophiuridae and Astrophytidae, new and old (Bull. Museum Comp. Zoology, Cambridge, 
III.), 1874. 
Ludwig, H., Beitriige zur Anatomie der Ophiuren (Zeitschr. fiir wissensch. Zool. XXXI.-XXXIYV.), 
1878-80. 
Ludwig, H,, Morphologische Studien an Echinodermen. Leipzie, 1877-79. 
Lyman, T., Report on the Ophiuroidea (Challenger Expedition, Zoology, vol. V.), 1882. 
Picard, K., Ueber Ophiuren aus dem oberen Muschelkalk (Zeitschr. d. deutsch. geol. Gesellsch. 
XXXVIII.), 1886. 
Boehin, G., Beitrag zur Kenntniss fossiler Ophiuren (Berichte d. naturf. Gesellsch., Freiburg, V.), 
1889. 
