134 MR. E. J. TILLYARD ON THE RECTAL BREATHING-APPARATUS 



A considerable range o£ variation exists in the long series of forms which 

 exhibit this type of gill. They do not, however, appear to be sufficiently 

 distinct, or of sufficient importance^ to warrant their elevation into the rank 

 of sub-types of the same order as 4 a and 4 6, though I have suggested 

 (p. 168) suitable names for them if it is desired to distinguish them in 

 that way. 



Methods. 



Three main methods of study have been followed, viz. : — 



(1) The making of ivJiole mounts of tlie branchial basket. 



(2) Photomicrography. 



(3) Microtomy. 



Whole Mounts. — Owing to the fact that the trachefe in the gills collapse 

 and lose their gaseous contents shortly after the death of the larva (usually in 

 from one to two hours after death), it seemed at first that all attempts at 

 making whole mounts must end in failure. However, after trying various 

 methods without success, some very excellent results were obtained as 

 follows : — 



The larva was killed by being dropped into a tube of water in which a 

 drop or two of chloroform had been well shaken up. It was then immediately 

 dissected lender xoater. The branchial basket, together with that part of the 

 tracheal system surrounding it (including the main trunks), was dissected 

 out, and opened up by a longitudinal cut near the mid-ventral line. It was 

 then plunged at once into Carl's Fixative (see under Microtomy, p. 136), 

 where it remained for 24 hours. After washing and dehydrating in the 

 usual manner, it was passed into equal parts of absolute alcohol and cedar-oil 

 for 24 hours, and thence into pure cedar-oil. There it remained for several 

 days, during which time the gills darkened slightly. Finally, it was mounted 

 on a slide within a raised ring, care being taken to remove as much cedar-oil 

 as possible, so that the Canada Balsam should dry properly. The whole 

 mount was then left for some weeks while it gradually darkened. The final 

 result shows the gill-formation in yellowish-brown or orange, with the 

 pigmented areas very strongly marked. The capillaries, of course, are not 

 visible, as they ha^'e collapsed. But for a study of the Simplex or Duplex 

 System in its entirety, these whole mounts are excellent. The drawings of 

 portions of the gill-basket in the various systems (text-figs. 2, 5, 9, 11, 12) 

 .are taken from photographs of some of these mounts. 



The everted gill-baskets are best mounted with their inner side uppermost, 

 so that the whole gill-structure can be clearly seen. If it is desired to study 

 the tracheal system, the slide may be turned carefully over and viewed from 

 the back". 



Pliotomicrography . — For this purpose the method followed previously by 



