Division in the Echinodermata. 335 



I do not know that any direct experiments have been made 

 on the divisibility and the faculty of regeneration in the Aste- 

 rida, although it is easy to prove that they are considerable 

 and general. They are not limited to the long-armed forms, 

 and are manifested in full energy in those which are almost 

 destitute of arms, such as the Asterince. It is sufficient that 

 out of five or six normal rays two are retained, in order that 

 the remainder shall be readily regenerated ; but from this 

 we must not deduce an absolute divisibility. Most of the 

 Asterida can, without difficulty, regenerate a lost fragment of 

 an arm from the surface of rupture itself, whilst in the species 

 of the genus Asterias (Aster acanthion) the disk alone is en- 

 dowed with this property, as Steenstrup has indicated. The 

 bifurcate (Y-shaped) arms, sometimes met with in various 

 Asterida*, may be attributed to an injury to these arms, just 

 as the double tails of lizards and the corresponding abnormal 

 formations in certain fishes (Syngnathi, Gymnotini) are due 

 to a lesion of the tail. The species of Asterias sometimes 

 present an arm which is bifurcated in a somewhat different 

 manner, a small branch or secondaiy arm issuing almost at a 

 right angle from near its extremity, with its ambulacrum 

 opening into that of the principal arm ; the origin of this ano- 

 maly is also undoubtedly a lesion produced at the point where 

 the lateral branch takes its rise "J". 



1856, M. Esmark indicated, in connexion with M. Steenstrup's communi- 

 cation, that he had likewise observed in Norwegian Asterida (no doubt 

 in Asterias rnbens) that an arm without disk had regenerated the deficient 

 parts. M. von Martens describes a comet-like specimen of Echinaster 

 eridanella (fallax) with six arms. 



* For example, Oreaster gigas and Astropect.cn aurantiacus. When 

 the bifurcation occurs near the origin of the arm, one might almost sup- 

 pose that the animal had two mouths. I know examples of this kind in 

 a small Asterina from the Mauritius and in Linckia multifora. When 

 the two points of union of the ambulacra are confounded in one, we have 

 six arms, two of which are united at the base (a Scytaster pisturnis in the 

 Museum presents this case). See Linck, ' De Stellis marinis,' tab. xiv. 

 fig. 24, and tab. xl. fig. 70: Seba, 'Thesaurus,' iii. tab. viii. fig. 9; and 

 Treviranus, Zeitschr. fur Physiol, iv. p. 124. 



t Before quitting this subject I may mention a peculiarity of Asterias 

 helianthus and some allied species (of the subgenus Heliaster). It has 

 already been said that we must' not interpret the heteractinism observed 

 in some Asterida and Ophiurida as due simply to the fact that all the 

 arms proper to the species do not make their appearance originally, but 

 only a portion of them, the deficient portion being subsequently deve- 

 loped ; and I added that, if such a case occurred among certain Echino- 

 derms with many arms, it would be much more natural to suppose that 

 the new arms (rays) would originate alternately with the old ones than 

 that they should all spring from a single point. From the fact that we 

 find three long and three short arms alternating with each other in a 



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