VAiiiotJS INVERTEBRATE ANIMALS. 115 



\vhole into spirit are almost useless for anatomical investigation. 

 If they can be made to expand in a basin of sea- water, and are then 

 plunged into a mixture of chromic acid and strong acetic acid, or 

 into strong spirit or corrosive sublimate, the structure of the polyps 

 is preserved. 



In preserving these forms, however, it is important to bear in 

 mind that, if two or three slices about | or ^ inch in thickness 

 are preserved separately, they afford the most valuable material for 

 investigation and study. Five per cent, formalin gives satisfactory 

 results if the polyps can be induced to expand in a basin of sea- water 

 after captui-e. 



Neai-ly all the Gorgonians have such a dense covering of spicules 

 that preserving fluids cannot penetrate into their tissues. We are 

 consequently almost entirely ignorant of the canal system of any 

 but the most common species. 



When a Gorgon ian is captured, therefore, in the dredge, a part of 

 the " bai'k " shoukl be stripped off and preserved separately in strong 

 spirit, or allowed to remain attached to the axis, if the spirit in 

 which the specimen is preterved can be kept of good strength, i.e. not 

 less than 70 per cent. 



The Pennatulids can be fairly well preserved by immersion whole 

 into spirit ; but it is always advisable, if more than one specimen 

 of a species is obtained, to make a long incision into the axis with 

 a knife before plunging into spirit, and any pinnae which can be 

 snipped off with a pair of scissors and preseived separately yield 

 good material for anatomical study. 



ECHINODERMA. 



Starfish may be prepared with the ambulacral feet in full dis- 

 tension by allowing them to die in from 20 to 30 per cent, alcohol. 

 Echinoids should be placed in a small quantity of water, and killed 

 with chrom-acetic mixture No. 2, being removed from it as quickly 

 as possible, as the acid corrodes the test. To preserve the internal 

 parts, it is necessary to make two opposite openings in the test, so 

 that the alcohol may penetrate the interior readily. 



