CLASS I ASTEKOIDEA 245 



character, as Gregory has claimed. In Ophiurans, however, it is almost 

 certain that alternation is a primitive and very important character. The 

 ventral position of the madreporite in Paleozoic Starfishes and the absence of 

 mouth shields in Paleozoic Ophiuroids point to an intimate relationship 

 between the two groups ; and this inference is still further confirmed by our 

 knowledge of several recent and fossil intermediate forms (Astrophiura, Pro- 

 taster, Ophiamhix, etc.). 



If one places a Starfish 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 Asterozoan 

 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, so far as at present known, reveals nothing definite in regard 

 to their close relationship. 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 quite 

 distinct from the other, as far back as the Cambrian. 



While it is clear that a well-marked separation exists between the two 

 classes of Asteroidea and Ophiuroidea, there is very unsatisfactory evidence 

 in support of a third group of equal rank, such as Ophiocistia Sollas or Aulu- 

 roidea Schondorf. The genera composing the former may better be retained 

 among the Ophiurans, while as for the latter, it is difficult to believe that 

 the characters assigned to it are real. More likely these supposed characters 

 rest upon a misinterpretation of the material. Fossil Asterozoans seem to 

 have been preserved in many cases only after the decay or removal of much 

 or all of the non-calcareous parts. Sometimes apparently the entire abactinal 

 side has been destroyed. In other cases only impressions remain, chiefly 

 of the harder parts, and the actual structure cannot be ascertained. It 

 is not surprising, therefore, that our knowledge of the Paleozoic forms 

 is still incomplete, and that erroneous interpretations should have been placed 

 upon some of their structural characters. 



Class 1. ASTEROIDEA Burmeister. Starfislies.i 



Asterozoans whose simple and more or less flattened arms are prolongations of 

 the central disk, and contain the hepatic appendages of the alimentary canal, as ivell 



1 Literature: Forhes, E., British Fossil Asteriadae. Mem. Geol. Survey, vol. ii., Part ii., and 

 Decade iii., 1848 and \SbO.— Salter, J. IF., New Palaeozoic Star-Fishes. Ann. Mag. Nat. Hist., 1857, 

 vol. XX. — Gray, J. E., Synopsis of the Species of Star-Fish in the British Museum, 1866. — Simono- 

 luitsch, S., Ueber einige Asteroiden der rheinischen Grauwacke. Sitzungsber. Wieu. Akad., ^^871, 

 vol. Ixin.Sars, G. 0., Researches on the Structure, etc., of the genus Brisinga. Christiana, 1875. — 

 Perrier, E., Revision de la collection des Stellerides du Museum d'Hist. Nat. de Paris. Arch, de 

 zool. experim., iv., v., 1875-76.— Agassiz, A. North American Star-Fishes. Memoirs Museum 



