i 9 6 DISCOVERY REPORTS 



see this in section as it rarely lifts away from the ectoderm cells. Its presence, however, 

 can be demonstrated by staining with chlorazol black, and this stain shows, further, that 

 the cuticle varies in thickness, being thicker towards the posterior margin bordering the 

 caudal furca. In the thinner anterior part it stains a purplish grey, while posteriorly 

 it takes on a definite greenish tint like the sclerites. 



The structure of the ectoderm itself can be seen through the musculature in prepara- 

 tions of parts of the body wall stripped off and mounted fiat, but in one fortunate 

 specimen I found it possible to dissect off the ectoderm alone without disturbing the 

 underlying musculature. It showed the typical appearance of pavement epithelium, 

 with extremely flat nuclei, but also at one point, a peculiarity to which I refer later in 

 connexion with the probable origin of the circular muscles. 



The arrangement of the circular muscles is such that they radiate approximately 

 from the attachment of the adductor muscles (Fig. 9). They appear to emerge gradually 

 from the lateral ectoderm and disappear similarly into the dorsal ectoderm on either 

 side of the middle line. At their dorsal ends they show typical structure, but ventro- 

 laterally Krause's membrane disappears, and more lateral still the myofibrils become 

 continuous. At their ectodermal attachments the fibrils appear to spread out into the 

 ectoderm cells as if the muscle were frayed. 



The relation of the circular muscles to the other components of the body wall can 

 be made out very clearly from the thick celloidin sections of adult specimens, but even 

 better in a thin section of an advanced embryo. 1 Here the circular muscle appears to 

 lie along the summits of rows of pyramidal ectodermal cells whose tops project inwards 

 (Figs. 2a, b). In fact, from the position of the nuclei, the ectoderm cells must be 

 arranged in this young stage in rows corresponding to the circular muscles. This linear 

 arrangement is lost in the adult and the nuclei become flattened out so as to be difficult 

 to see in a section of the ectoderm, but the connexions to the circular muscles remain 

 as fan-shaped bundles of fibres. The two figures (Figs, za, c) illustrating this point are 

 parasagittal sections which naturally cut across the circular muscles. 



In a transverse series, that is, one which includes a section in the plane of the adductor 

 muscle, it is always possible to find a section which includes the whole of a circular 

 muscle. Fig. zb shows such a section. Here the muscle can be seen lying close against 

 the inner face of the ectoderm cells and then extending gradually at either end into 

 a group of ectoderm cells which are stretching out towards it. The fibres upon which 

 the cross-striations will subsequently develop run as a distinct band up to the point 

 approximately where the muscle branches out into terminal connexions, but there does 

 not appear to be any definite boundary between the two zones. 



In the adult the termination of the muscle by its merging into the ectoderm is even 

 more marked. The striations cease near the converging ectoderm cells, but fibrillae 

 extend into these cells to the cuticle itself. These can best be seen in a preparation of 

 the body wall mounted flat. The individual fibres stain intensely in chlorazol black and 



1 Gigantocypru carries its eggs inside its shell (Plate XXXIX, fig. 10). It is peculiar in that the eggs do not 

 hatch as a nauplius but as a young adult. 



