ON A FIIEE-SWIMMING HYDEOID. 



another layer of muscle-fibres, which we may consider to be 

 endodermal in origin. These fibres are arrauged in a 

 circular manner at right angles to those of the ectoderm (fig. 

 10), and the extent of the muscular layer is increased by 

 horizontal folds, similar to the vertical folds of the ecto- 

 dermal layer. These horizontal folds are, of course, recog- 

 nisable only in vertical sections, while the vertical folds of 

 the ectodermal musculature are conspicuous in transverse 

 section (fig. 10). 



The free surfaces of the gastral ridges bounding the 

 gastral cavity are covered with an epithelium of a very 

 peculiar type (figs. 9 — 11). It consists of long, slender, 

 columnar cells arranged at right angles to the surface. 

 They have a finely granular cytoplasm and distinct nuclei, 

 and appear in the sections to be collected into small groups, 

 like bundles of cigars, from the inner ends of which delicate 

 wavy fibres run obliquely towards the central plane of the 

 ridge, and thence inwards side by side till they meet the 

 mesoglceal supporting lamella, where they probably give rise 

 to the circular musculature.^ The grouping of the epithelial 

 cells into bundles is, I think, probably a post-mortem con- 

 dition due to contraction in alcohol. I imagine that the cells 

 are normally arranged so that each is continued inwards 

 into a separate fibre. We may probably regard the endo- 

 derm of the gastral ridges as glandular-muscular in function, 

 for no doubt it secretes the digestive fluid. There are no 

 thread-cells in the gastral ridges, nor, indeed, have I seen 

 them in any part of the eudoderm. On approaching the 

 annular endodermal fold which marks the upper limit of the 

 proboscis the gastral ridges gradually die away, and their 

 epithelium gives place to that which lines the gastral face of 

 the septum on the one hand, and the endodermal canals on 

 the other (fig. 8). 



The mesoglceal supporting lamella of the proboscis wall 

 may be regarded as being bounded on the outside by the 



' Compare t.lie structure of the endodermal villi willi tlicir muscle-fibres iu 

 M}' riot lie la (Hardy, ' Quart. Jouru. Micr. Sci,,' vol. xxxii, p. 5U5). 



