900 ECHINODERMA 



spines, evolves itself into its particular generic and specific type, 

 and undergoes various changes of internal structure tending to 

 the development of the complete organism. 1 



An excellent summary of the developmental history of the 

 several Echinoderm types, with references to the principal memoirs 

 which treat of it, will be found in Chapter XX. of Mr. Balfour's 

 ' Comparative Embryology,' and in Professor A. Lang's ' Jahrbuch 

 der vergleichenden Anatomic,' which has been translated into English. 2 

 In collecting the free-swimming larvae of Echinoderma the stick- 

 net should be carefully employed in the manner already described, 

 and the search for them is of course most likely to be successful in 

 those localities in which the adult forms of the respective species 

 abound, and on warm calm days, in which they seem to come to the 

 surface in the greatest numbers. The following mode of preparing 

 and mounting them has been kindly communicated to the Author 

 by Mr. Percy Sladen : ' For killing and preserving echinoderm zooids, 

 I have come to prefer either osmic acid or the picro-sulphuric mix- 

 ture of Kleinenberg of one-third strength. The latter, of course, 

 destroys all calcareous structures ; but the soft parts are preserved 

 in a wonderful manner. If the diluted Kleinenberg's mixture is 

 used, let the zooids remain in it for one or two hours; then wash 

 them thoroughly in 70 per cent, spirit, until all trace of acid is re- 

 moved ; then stain ; then again wash in 70 per cent, spirit, transfer 

 them to 90 per cent, spirit for some hours, and lastly to absolute 

 alcohol. Transfer them from this to oil of cloves ; and finally mount 

 in Canada balsam in the usual manner. If osmic acid be used, place 

 three or four of the living zooids in a watch-glass of sea-water, and 

 add a drop of the 1 per cent, solution. They should not remain even 

 in this weak solution for more than a minute, and should then be 

 thoroughly washed in a superabundance of 35 per cent, spirit, to pre- 

 vent the deposit of crystals of salt consequent on the action of the 

 osmic acid. Then transfer the specimens to 70 per cent, spirit, and 

 proceed as in the other case.' 



One of the most interesting to the microscopist of all Echino- 

 derma is the Antedon* (more generally known as Comatula), or 

 ; feather-star' (fig. 685), which is the commonest existing representa- 

 tive of the great fossil series of Crinoidea, or ' lily-stars,' that were 

 among the most abundant types of this class in the earlier epochs of 

 the world's history. Like these, the young of Antedon is attached 

 by a stalk to a fixed base, part of which is shown in fig. 686 ; but 

 when it has arrived at a certain stage of development it drops off from 

 this like a fruit from its stalk, and the animal is thenceforth free to 

 move through the ocean water it inhabits. It can swim with con- 



1 Abbreviated development, in which there is no free-swimming larva, is now 

 known to be more common than was once supposed : among Holothurians Cncu 

 maria crocea, among Ophiuroids Ophiacantha vivipara, and among Echinoids 

 Hemiaster cavernosus may be cited as examples. 



2 Those who wish to carry their study further must consult the recent memoirs 

 of Mr. Bury, Prof. MacBride, and Dr. Willey, and that of Dr. T. Mortensen, Die 

 Echinodermenlarven der Plankton Expedition (Kiel and Leipzig, 1898), in which 

 there is a systematic revision of the Echinoderm larvae already known. 



5 See the Author s ' Researches on the Structure, Physiology, and Development 

 of Antedon rosaceus,' Part L, in Phil. Trans. 1866, p. 671. 



