140 MR. R. J. TILL YARD ON THE RECTAL BRBATHING-APPARATDS 



be seen sLx radial strands o£ muscle arising in the folds of the thin epithelium 

 separating the so-called "rectal glands," and passing out across the hsemo- 

 coele to become attached to the body-wall. These are the dilatator muscles of 

 the rectum. They not only serve in a sense as actual supports for suspending 

 the posterior part of the rectum firmly in the hgemocoele, but, by their con- 

 tractions, cause the rectum to expand, and thus allow of the entry of water 

 through the rectal valves into the gill-basket. 



In the later larval stages the presence of these muscles gives the posterior 

 portion of the gill-basket a characteristic hexagonal shape in cross-section. 

 I'urther forward this effect is not noticeable, the usual shape of the basket 

 being nearly circular or slightly compressed dorso-ventrally. 



Using the clock-face notation, the positions of the insertions of the dilatator 

 muscles are 1, 3, 5, 7, 9, 11. The positions of the rectal glands are 2, 4, 

 6, 8, 10, 12. These facts are of great importance when we come to the 

 discussion of the homologies of the rectal gills. 



C. Its General Structure. 



All the gill-baskets of Anisopterid larvse show the following general 

 structure : — 



Passing from within outwards we meet first with a fine cuticle or chitinous 

 intima. External to this is the rectal epithelium, from which the cuticle 

 is derived. Surrounding the epithelium is an almost continuous layer of 

 circular muscle-Jihres only one row thick, with numerous openings for the 

 entry of the tracheas into the gill-basket. Finally, external to this, we can 

 recognise a layer of longitudinal muscles, not forming a tunic, but segregated 

 into six rather weak longitudinal hands, each formed of a small number 

 of separate fibres. These are placed in the positions 2, 4, 6, 8, 10, 12 of the 

 clock-face. 



The Cuticle. — This is a very fine, transparent, non-cellular, non-staining 

 layer which covers the internal face of the rectal epithelium throughout. It 

 is absolutely smooth and structureless except along the edges of the gill-folds, 

 where it is usually armed with a series of tiny chitinous teeth. It is secreted 

 by the rectal epithelium, and is cast off at each ecdysis. It is exactly com- 

 parable with the similar fine internal cuticle which lines internally the 

 stomatodseal portions of the alimentary canal. 



The cuticle forms the outer covering of such organs as the papillge and 

 tubercles of the gills, when these are developed. In such cases it may 

 develop a special armature of strong spines or of transverse chitinous rods, 

 which will be described under the study of those types of gill in which 

 it occurs. 



The Rectal Epithelium. — This is remarkable in showing, throughout the 

 gill-basket, no trace whatever of separate cell-divisions. It is, in fact, a 



