200 ECHINODEBMATA ASTEROZOA SUB-BRANCH n 



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, etc.) 



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 Callocystidae). 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. 1 



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, wmk are sharply separated from the central disl^ 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 (Illustr. Cat. Museum Comp. Zoology, Cambridge. Nos. 



I.-IIL), 1865. 

 Lyman, T., Ophiuridae and Astrophytidae. new and old (Bull. Museum Comp. Zoology, Cambridge, 



III.), 1874. 

 Ludvrig, H., Beitrage zur Anatomie der Ophiuren (Zeitschr. fiir wissensch. Zool. XXXI.-XXXIV.), 



1878-80. 



Ludwig, H., Morphologische Studien an Echinodermen. Leipzic, 1877-79. 

 Lynian, T., Report on thr Ophiuroidea (Challenger Expedition, Zoology, vol. V.), 1882. 

 Picard, K., Ueber Ophiuren aus dem oberen Muschelkalk (Zeitschr. d. deutsch. geol. Gesellsch. 



XXXVIII.), 1886. 

 Boehm, G., Beitrag zur Kenntniss fossiler Ophiuren (Berichte d. naturf. Gesellsch., Freiburg, V.), 



1889. 



