CHAP, xvii.] VARIATION IN HOLOTHURIOIDEA. 435 



Now all this argument rests on the premiss that the several 

 members of a series of differentiated parts cannot undergo a 

 Substantive Variation in correlation with Meristic change in the 

 total number of members constituting the series. It is assumed 

 that there can be no redistribution of differentiation. 



This assumption has now in many cases of Linear Series been 

 shewn to be false. To refer to one of the simplest cases, there 

 is, in the case of the Frog, evidence that the peculiarities of the 

 9th vertebra may be wholly or in part transferred to the 10th 

 vertebra, when by Meristic Variation there are 10 vertebra? 

 (Nos. 56, 57 and 60), and the like has been shewn in many other 

 examples (cp. No. 35). The functions (as indicated by the struc- 

 tures) of the vertebra? may be redistributed on the occasion of 

 Meristic Variation. 



Will anyone affirm that similar redistribution of differentiation 

 may not happen in the Meristic Variations of Echinoderms ? 



543 Variations in organs of Holothurioidea. LAMPEBT calls attention to the 

 great variability found in this group and the consequent difficulty in distinguishing 

 specific characters from individual abnormalities. These variations often take the 

 form of alterations in the number of organs. For example, the distribution of 

 the tube-feet is liable to great alterations during the lifetime of individuals. In 

 some forms (as Thyone and Thyonidium) the feet are confined to the ambulacral 

 areas in the young animal, but are distributed over the whole body in more mature 

 individuals ; and in species of the genus Stichopus, though the tube-feet are 

 arranged in rows, yet in old individuals this arrangement may become obliterated. 

 On the contrary, in others, as for example, Holothuria graeffei, the arrangement 

 of the feet in thoroughly mature specimens is still most sharply denned. 



The number of the tentacles is generally a multiple of rive, and such cases 

 as Amphicychis and Phyllophorus in which other numbers are found, are rare. 

 In these forms the tentacles are said to vary both in number, position and size, 

 but the number is always about 20. The case of Thyonidium molle is cited as 

 an extreme case. Of this species 4 specimens had 20 tentacles arranged in a paired 

 manner as in typical Thyonidia ; other specimens had 20 tentacles of similar 

 length ; others had from 16 to 19 tentacles of nearly equal lengths, and others 

 again had from 19 to 21, which instead of being disposed in pairs were placed 

 irregularly, some being larger and some smaller. 



Of all the organs, the Cuvierian organs are the most variable and they are 

 of little value for purposes of classification. Their number is very inconstant 

 and they may even be absent altogether. It is impossible to distinguish any 

 circumstances whether of locality or of structure in which the individuals without 

 Cuvierian organs differ from the others which possess them. The two chief ap- 

 pendages of the water vascular ring, namely Polian vesicles [cp. No. 642] and the 

 stone-canal are usually constant when they are single, but in rare cases there 

 are exceptions even to this rule. If however more than one of these organs is 

 normally present, it may generally be assumed that there is no constancy in 

 their numbers, and in such cases the number of the Polian vesicles is especially 

 variable. A few species have been recorded in which, from a single Polian vesicle, 

 secondary ones are formed by lateral outgrowths. 



The calcareous plates are of all the organs the least liable to variations, but 

 in certain cases they are stated to change with age. 



LAMPEKT, K., Die Seewalzen, in Semper's Reisen im Archipel der Philippine!!, 

 1885, iv. in. pp. 6, 13, and 174; also in Biol. Centrabl. v. p. 102. 



CRINOIDEA. 



Variation from the pentamerous condition has been many times 

 observed, though considering the vast number of specimens collected 



282 



