ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
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organisms survive through rapidly changing physical conditions. When 
the changes of environment became too rapid, the forms either ceased 
to exist or retrograded, became depauperate, and finally extinct ; this may 
be illustrated by Batocrinus, Dorycrinus, and Dichocrinus. Variation may 
go on in one portion of an organism withoutmaterially atfecting other parts. 
The Actinocrinidae show a decided tendency throughout their existence 
to increase the distal extent of their rays. This was accomplished by 
simple branching of the free arms, as in Megistocrinus, by the lateral 
expansion of the arms, as in Eretmocrinus, or by radial extension of 
the calyx-brachials, as in Teleiocrinus and others. 
Ambulacral and Adamhulacral Plates of Starfishes.* — Mr. J. W. 
Fewkes comes to conclusions regarding the homologies of these 
plates, which differ from the generally received doctrines. He finds 
that there is no difference in the way the mouth-parts of typical 
representatives of the groups known as Asterim Ambulacrariae or 
A. Adambulacrarise are developed. The arm of a starfish is made up of 
somites, and the water-vascular system of vessels may be supj^osed to 
be primarily surrounded by a calcification. The theoretical ring of 
calcification is most closely reproduced in its typical form in the plates 
surrounding the mouth. The ambulacrals and adambulacrals are 
l^ortions of the annular calcification of successive segments, and are 
serially homologous. The ambulacrals of starfishes are not represented 
in sea-urchins except around the mouth, when they appear as auricles. 
The adambulacrals of starfishes represent the ambulacrals of sea-urchins 
and complete the external portion of the problematical ring of calcifi- 
cation, which is absent in Asteroids. The marginal plates of Asterias 
are homologous wfith the so-called adambulacrals of sea-urchins. 
With regard to spines it seems to Mr. Fewkes to be necessary to 
distinguish the ordinary spines of the adamhulacral arm-plates, the 
hook-shaped spines found on each side of the terminal in some young 
Ophiurids, or on the adults of others, the fins of Ophiopteron, and the fan- 
shaped spines of Asterias. 
French Holothurians.l — M. E. Herouard gives an account of the 
Holothurians found on the coast of France. He regards a Holothurian 
as an Echinoderm whose plane of symmetry does not correspond to 
that of the Spatangoids; the left ventral radius of the latter is the 
homologue of the median ventral radius of a Holothurian. The inter- 
radius which corresponds to the madreporite of Echinoids is, therefore, 
in the median dorsal line. 
The integument presents . three zones, the innermost of which is 
muscular ; the intermediate one is formed of an inner nervous and an 
outer connective layer, and belongs to the “ amoebophorous system ” ; the 
outermost layer is connective, is very strong, and contains the calcareous 
corpuscles ; it plays the part of a protective organ like the test of an 
Echinoid. In some species there is a circumanal apparatus, the radial 
plates of which are the homologues of the ocular plates of Echini. 
The calcareous corpuscles are always formed of a hexagonal plexus, and 
* Proc. Boston Soc. Nat. Hist., xxiv. (1889) pp. 96-117. 
t Arcl). Zool. Exper. et Gen., vii. (1889) pp. 535-704 (8 pis.). 
