ECHINODERMATA: HOLOTHURIOIDEA. 549 



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, &c., 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 Holo- 

 thurioidea, Echinoidea, Ophiuroidea, Asteroidea, Crinoidea and the wholly 

 extinct Cystoidea and Blastoidea. The Holothurioidea are sometimes 

 termed Scytodermata ; the Echinoidea, Ophiiiroidea 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 Crinoidea, 

 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, &c., 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. 



CLASS HOLOTHURIOIDEA s. SCYTODERMATA. 



Echinodermata with elongated bodies, and a tough integument with well 

 developed muscular coats, and as a rule a feebly developed calcareous skeleton. 

 There is a circle of circumoral tentacles borne by the water-vascular ring, 

 and the madreporic tubercle is usually placed internally. The mouth is 

 ventral or terminal ; the anus ventral, terminal, or dorsal. 



The body is often more or less vermiform, or in cross section pen- 

 tagonal. In other instances one surface corresponding to three ambulacral 



