300 PROCEEDINGS OF THE ACADEMY OF [1887. 



breadth and thickness in various parts of the body, as do the ecto- 

 dermal cells. The entodermal cells are raised up into two ciliated 

 bands upon the lateral walls of the branchial chamber and form a 

 hoop-like elevation about its anterior end, which being inclined back- 

 ward as it passes toward the dorsal surface, connects the anterior 

 ends of the endostyle and the gill. 



Between the ectoderm and entoderm is a transparent structure- 

 less material of the same appearance as that composing the outer 

 material, but lacking the elasticity of the latter on account of its 

 being pierced and hollowed out by the numerous blood channels 

 and sinuses. Lyiug embedded in this porous matrix are the 3fus- 

 cles ; these are composed of from six to twelve broad, flat, striated 

 muscular fibres arranged in bundles, with their broad surfaces in 

 contact and their edges presenting towards the interior and exterior 

 of the body. (figs. 3, 4, 5.) The fibres are made up of several 

 large muscle cells which have become fused together, each fibre 

 shomng a large number of oval nuclei, clear, bladder-like, with rel- 

 atively large nucleoli. The fibres have a longitudinally striated 

 appearance caused by the granular contents being arranged in rows 

 rej^resenting the ultimate fibrillae. The transverse striation is not 

 always to be seen, but there is usually present, especially when the 

 bundles are viewed on the surface, an irregular transverse marking 

 of the entire bundle, due to certain portions of the cells taking a 

 deeper staining, (fig. 5.) 



The Gill ("hypopharyngeal band" Huxley) is a cylindrical tube 

 in the living animal, but in preserved specimens more or less col- 

 lapsed. Its walls are a continuation of the entoderm, and it is filled 

 with the same spongy basis material that separates the ectoderm 

 and entoderm, and like this it is perforated by an irregular series 

 of blood sinuses ; not by "a single grand sinus" as described by 

 Huxley^ (fig. 6, 7.). The cells constituting the walls of the gill are 

 in the main, identical with those of the entoderm and remain unchan- 

 ged along the upper and lower surfaces of the organ, but on the sides 

 they become altered into two longitudinal series of ciliated ribs 

 (e h, fig. 6.). These form cushion-like elevations, and are made up 

 of three layers of spindle-shaped cells, the outer layer of which bear 

 rather long stiff cilia. 



The cilia-bearing cells are arranged in regular rows upon the cush- 



1 Huxley, (F. H.) : On the Anatomy of Salpa and Pyrosoma. Royal 

 Society Transactions, 1851, p. 570. 



