21 



pair of arms, which are without tentacles. In young males, the epidermis of the arms, except 

 for a short region at the base, is entirely filled with refringent vesicles (PI. IX, fig. 99), 

 similar to those which occur in the terminal bulbs of the arms of C. dodccalophus and of the 

 young C. gracilis. These vesicles appear to have been used u]) in some of the old males. The 

 epidermis of the male, as in the case of the neuter, generally contains a great quantity of 

 pigment. The body is large, and is almost entirely filled by a pair of enormous testes. The 

 alimentary canal is vestigial. 



VI. METHODS AND GENERAL ANATOMY. 



In this Section I give a short description introductory to the Sections dealing with the 

 special anatomy of the regions and organs. 



My study of the structure of Cephalodiscus has been greatly facilitated by the use of 

 solid reconstructions made from series of sections. The method which has been of most service 

 is the one which was introduced by my friend Professor J. Graham Kerr (01, p. 5) and 

 improved in some particulars by the late Mr. J. S. Budgett (02, p. 318). The principle of the 

 method consists in drawing the sections on plates of finely ground glass, of uniform and suitable 

 thickness, and immersing the superposed plates in a liquid having the refractive inde.x of the 

 glass. The liquid employed was a mixture of fennel-oil and cedar-wood-oil, as recommended by 

 Budgett ; and the proportions of the mixture which I have found it convenient to use are 

 two parts of unthickened fennel-oil to one part of cedar-wood-oil. In this way a transparent 

 reconstruction is obtained, in which different organs are brought out if they have previously 

 been coloured with ordinary water-colour paints. Opaque colours, such as vermilion, can be 

 used where it is desired to see only the outer form of an organ ; and transparent colours in 

 cases in which it is necessary to see internal details. Figs. 22 — 24 are in the main from 

 reconstructions prepared in this way. 



Kerr's method can be most strongly recommended for many purposes; but finding that 

 in some complicated cases, a more tangible amount of solidity was required in a reconstruction, 

 I turned my attention to a form of modelling clay known as "plasticine". This clay has the 

 advantage of not drying up, and of thus retaining its softness for any length of time. Thin 

 plates of the plasticine can readily be prepared by pressing the substance on a smooth surface 

 (such as a piece of paper), and removing the irregularities from the upper side of the plate 

 by means of the back of a scalpel. The sections are drawn with a camera lucida on thin 

 paper, and their outlines cut out with .scissors. The paper pattern is pressed on the plasticine 

 plate, which is then cut to the shape of the section with the point of a mounted needle. 



The plates are superposed (removing the paper from the lower plate just before adding 

 a new one), taking care to smooth off the edges and to fill up irregularities after the addition 

 of each plate. The completed reconstruction, if carefully made, is a faithful reproduction of 

 the original. The method does not differ in principle from the wax-plate method which has 

 previously been used; but among its advantages are its great cheapness, and the ease with 



