ASTKKOIDEA. 



Echinodermata. 



737. Holothuroidea. These animals are difficult to fix, on 

 account of their contracting with such violence under the 

 influence of irritating reagents as to expel their viscera through 

 the oral or cloacal aperture. It has been recommended that 

 they be seized by the middle of the body and firmly squeezed 

 in the hand, and so plunged in a fixing liquid (acetic acid, for 

 instance) ; or that they be anaesthetised (in the case of Synapta 

 and Cucumaria) by adding ether to the water in which they 

 are contained. So far as my experience goes, I am bound to 

 say that I know no better way of killing them than that of 

 simply putting them into fresh water, in which they generally 

 die without contraction and with their tentacles extended. 



VOGT and YUNG (Anat. Comp. Prat., p. 641) say that Cucu- 

 maria Planci (C. doliolum, Marenzeller) is free from the vice 

 of expelling its intestines under irritation ; but they recom- 

 mend that it be killed with fresh water, or by slow intoxication 

 with alcohol, chromic acid, or sublimate added to the sea- 

 water in which it is contained. 



738. Asteroidea. There are great difficulties in the way of 

 fixation here too. It is quite possible to obtain a fixation of 

 the ambulacral feet, branchiae and tentacles in the extended 

 state, by throwing the animals into boiling water, and then 

 bringing them into a fixing liquid. But this method has the 

 fault that the fixing liquid so employed only penetrates 

 extremely slowly into the interior of the animal, and therefore 

 does not give a good fixation of internal organs. 



HAMANN (Beitr. z. Hist. d. Echinodermen, ii, 1885, p. 2) finds 

 it preferable to inject the living animal with a fixing liquid. 

 The canula should be introduced under the integument at 

 the extremity of a ray, and the liquid injected into the body- 

 cavity. The ambulacral feet and the branchiae are soon dis- 

 tended by the fluid ; and as soon as it seems to have penetrated 

 sufficiently, the animal is thrown into a quantity of the same 

 reagent. 



The study of the eyes present points of special difficulty. 

 In order to study them in sections, with the pigment preserved 

 hi *itu, the eye should be removed by dissection, should be 

 hardened in a mixture of equal parts of 1 per cent, osmic acid 

 and 1 per cent, acetic acid, and imbedded in a glycerin gum 



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