A sexual difference in colour has been noticed in the Asteroid Oreaster (Pentaceros) turritus and Ophiuroid Ophiothrix Petersi. Otherwise the sexes are alike.
Among the Holothurioidea the majority mutilate themselves by discharging their Cuvierian organs. The Aspidochirotae also break off the alimentary tract behind the water-vascular ring but repair the injury. If irritated, the Synaptae break the body up into sections and certain species of Stichopus resolve their skin into a diffluent mucus. Asteroidea, Ophinroidca and Crinoidea regenerate an injured arm, and the two latter, especially the Ophiuroidea, possess the power of throwing the arms off. In the Asteroidea and Ophiuroidea the organism may be formed anew from an arm with a portion of the disc or perhaps even without it. It is possible indeed that asexual multiplication may thus take place normally in some genera. The Crinoidea appear to have the power of regenerating the visceral mass as a whole when removed or lost naturally as it sometimes is.
Segmentation of the ovum is total. The gastrula is formed as a rule by invagination. It is stated to be delaminate in the Ophiuroids Ophiothrix versicolor and Amphiura squatnata. The mesoblast is derived from amoeboid cells which originate from the invaginated cells (i. e. hypoblast), and the epithelium of the coelome and water-vascular vessels from the vaso-peritoneal vesicles, hollow outgrowths of the archenteron, solid only in certain Ophiuroidea. The larval oesophagus is a stomodaeum. The larva is at first uniformly ciliated, but the cilia are subsequently restricted to certain bands. Antedon has four transverse bands and a posterior tuft. In all other Echinoderms, with the exception of a few with shortened development, the free-swimming larva has a prae-oral and a prae-anal ciliated ridge. These ridges either unite into a single closed longitudinal band (Holo-t/uirioidea, Echinoidea, Ophiuroidea) or form two separate but closed bands of which the prae-anal is longitudinal (Astej'ioidea). The Holothurioid larva is known as Auricularia. Its ciliated band is somewhat undulated and breaks up into transverse ciliated bands, usually five in number, constituting the so-called Pupa-stage. The prae-oral lobe is large.
The Asteroid larva closely resembles the Holothurioid. Its ciliated band developes soft arms, and it is then known as Bipinnaria; or as Brachiolariay if three special arms covered with warts are also developed at the anterior dorsal extremity independently of the ciliated band. The Echinoid and Ophiu-roid larva is known as Pluteus. It is distinguished by the small size of the prae-oral dorsal area, and the large size of the post-anal area, and by the growth in the course of the ciliated band of arms directed anteriorly, and supported by a provisional calcareous skeleton. The arms common to the Pluteus of both classes are, a pair of anterior or oral arms, a pair of posterior or prae-anal arms, and a pair of antero-lateral arms. All these processes, etc, are secondary characters, and are absorbed during the metamorphosis of the larva into the adult.
The Echinoderms are marine; a few Holothurioidea, however, can . live in brackish water. They may be grouped into the classes Holothurioidea, Echinoidea, Ophiuroidea, Asteroidea, Crinoidea and the wholly extinct Cystoidea and Blastoidea. The Holothurioidea are sometimes termed Scytodermata; the Echinoidea, Ophiuroidea and Asteroidea may be grouped together as Echinozoa, and the remaining three classes as Pelmatozoa.
It has recently been asserted that the water-vascular and blood-vascular systems are closely connected in Echinoidea and Crinoidea. The work of Ludwig on. Asteroidea and Ophiuroidea, and of Ludwig and P. H. Carpenter on Crinoidear appears to settle the question in the negative, so far as those groups are concerned. It is probable also that the Echinoidea will be found to conform to the same type of arrangement; cf. P. H. Carpenter, Q. J. M. xxiii. 1883, and xxv. 1885.
For the interradius of the water-pore, see Ludwig, Z. W. Z. xxxiv. 1880.
In Asterina gibbosa the larval mouth and oesophagus are closed, and a new mouth is formed in relation with a new oesophagus, which grows out from the archenteron to the left side of the larva. The larval anus (= gastrula mouth) is also closed, and the anus of the adult is a new formation. Ludwig, Z. W. Z. xxxvii. 1882.
For the formation of the arms, etc, in the Starfish just named, see p. 312, ante.
Sexual Dimorphism, Studer, Z. A. iii. 1880, p. 523.
Suckers, Niemiec, Recueil Zool. Suisse, ii. 1885.
Fossil Echinodermata, Zittel, Handbuch der Palaeontologie, Abth. I, Palaeo-zoologie, i. pp. 308-560.