THE STELLEROIDEA 273 



Trichaster the stone-canals are thus repeated, but there is only one 

 madreporite. A repetition of the stone -canals also occurs in 

 Ophiactis, in which the value of this character is apparent, for the 

 animal reproduces by fission ; in the same genus there are also 

 several Polian vesicles in each interbrachial space. In development 

 the water-pore of recent Ophiuroids originally opens on the dorsal 

 surface (Bury, 6, pp. 422, 423, pi. xxxvii. f. 2), and then works round 

 to the ventral side, where it becomes attached to an oral plate. 



The Alimentary System remains very simple in all the Ophiuroids, 

 consisting simply of a large chamber, divided into a series of short, 

 blunt, sac-like protuberances by radial constrictions of the walls. 



Respiration in the Ophiuroids is generally effected by the 

 genital bursae and the podia ; but when the bursae are absent, 

 their place may be taken by an extra series of Polian vesicles, as 

 in Ophiactis ; or the general body-cavity may be used both for the 

 protection of the ova and for respiration, as in Gorgonocephalus. 



Reproduction. The genital bursae in some Ophiuroids also act 

 as brood chambers ; the eggs pass through all stages of development 

 in them, and such Ophiuroids are therefore viviparous ; Amphiura 

 squamata and Ophiomyxa vivipara are examples of this condition. 

 Asexual reproduction occasionally occurs in Ophiuroids either 

 normally by fission, as in Ophiactis, or abnormally by regeneration 

 of lost parts when the disc of an Ophiuroid is cut into halves. 

 Broken arms are readily replaced, but a broken arm cannot 

 reproduce a complete animal as can be done in the Asteroids 

 (Semon and Ludwig). 



The Development of the Ophiuroids agrees in the early stages 

 with that of the Asteroids, but the larval form is a Pluteus (p. 6) 

 and not a Bipinnaria (p. 5). It, therefore, in this stage offers a 

 greater resemblance to the larvae of the Echinoids than of the 

 Asteroids. There is no doubt, however, that this larval form is 

 a secondary development and does not represent any stage in 

 phylogeny of the group ; it therefore does not indicate any 

 affinity with the Echinoids. 



Distribution. The Ophiuroids range from shallow and estuarine 

 waters to abyssal depths. Their distribution in space is wide, 

 species such as Ophiocten sericeum occurring at 80 N. latitude ; but 

 the largest forms are tropical. The order was first represented by 

 species of Protaster from Ordovician rocks. Representatives of the 

 Lysophiurae and Streptophiurae occur in the Silurian, Devonian, 

 and Carboniferous strata ; Zygophiurae begin in the Trias ; and 

 Cladophiurae in the Jurassic. 



On the characters discussed in the preceding pages may be based the 

 following Diagnosis of the Sub-Glass : J The Ophiuroidea are " eleuther- 

 ozoic," " actinogonidial," and " lysactinic," Echinoderms which usually 

 1 Emended from Bell (4), p. 215. The terms are explained antea, p. 287. 



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