22 GENERAL FEATURES, METHODS, ETC. 



and the walls of the median undivided section and a portion of the canals leading 

 on to the pericardium become greatly thickened to form the shell gland. 

 Whether these canals ever function as kidneys is an unsettled question. In 

 the Chaetodermatidae there are indications that they do, but at the present 

 time there is no experimental evidence in support of such a view. 



Our knowledge of the development of this group of animals is very incom- 

 plete. From Pruvot's work on Myzomenia banyulensis, and my own on Halo- 

 menia, it is evident that the early history resembles that of certain lamellibranchs 

 and the scaphopods. Pruvot's interesting discovery of a stage where the embryo 

 bears seven calcareous shell plates indicates, as a number of authors maintain, 

 that the Aplacophora have descended from polyplacophorous ancestors. 



Methods. — On several occasions I have tried the effects of various killing 

 fluids, and am convinced that for general histological work alcohol is the most 

 satisfactory and is easily controlled. Specimens from deep water are usually 

 in a moribund condition when they come up in the dredge, and undergo prac- 

 tically no contraction when plunged directly into 70% alcohol. Where the 

 animals are more hardy or have been taken in comparatively shallow water it 

 is advisable to add gradually chloretone (aceto-chloroform) dissolved in alcohol 

 until they are completely narcotized. They may then be placed in 70% alcohol 

 for a few hours and preserved permanently in an 80% solution. In warm 

 weather it is sometimes necessary to keep the specimens in a cool place until 

 thoroughly fixed, and in any case it is necessary to use considerable quantities 

 of alcohol. In the study of the nervous system I have found vom Rath's fluid 

 highly satisfactory especially when the material is treated subsequently with a 

 1% solution of pyroligneous acid. When sufficiently oxidized the nerve fibres 

 remain grayish in color and are usually quite distinct among the yellow muscle 

 and connective-tissue fibres. For staining I have generally used Delafield's 

 haematoxylin, rarely using rubin as a secondary stain. 



In connection with certain features of the nervous system the specimens 

 used were of sufficient size to allow of dissection. Under such circumstances 

 paraffine was poured into a small dissecting pan and while it remained soft the 

 mollusc was partially imbedded in it, thus obviating the use of pins. Dissection 

 was done under alcohol by means of a needle mounted on the end of the arm of 

 an instrument for mounting diatoms. 



In removing the cuticle from about the spicules eau de Javelle is preferable 

 to caustic potash which frequently exerts a decided corrosive action on the more 

 delicate spines. 



