NOTE ON THE CILIATED PIT OF ASCIDIANS. 139 



of two kinds, one appearing dark brown or black, and the other 

 a bright orange. 



Body Wall axd Body Cavity. 



Beneath the test and closely applied to it is a layer of 

 columnar cells (figs. 12 and 15, Ep.) with very definite nuclei. 

 Their internal ends are rounded aud they do not rest upon any 

 basement membrane. 



Beneath the epidermis is a thin layer of circular muscle- 

 fibres (fig. 15, c. m.), then a thin layer of longitudinal fibres 

 (fig. 15, /. m.). Within the latter is a very thin layer of cir- 

 cular fibres (fig. 15, c. m.), and then a thick layer of longitu- 

 dinal and oblique fibres (fig. 15, I. m.), which are arranged in 

 large irregular masses, not definitely marked off from one 

 another, or from the surrounding tissue (fig. 15). All the 

 muscles are unstriated. 



A mass of connective tissue, constituting a meshwork of 

 fibres, fills up the spaces between the muscles and extends 

 inwards as far as the liuing of the atrial cavity (figs. 12 and 15 

 Ps.). Nuclei are sometimes visible (fig. 15, c. t. c.) at the 

 points of junction of the fibres ; these apparently are the 

 nuclei of the connective-tissue cells, the cell protoplasm beino- 

 so drawn out and branching as not to be apparent round them. 



Among the meshes of the connective tissue and lving freely 

 in it are three kinds of cells : 



1. Cells filled with black pigment (fig. 15, p. c), A nucleus 

 is present in each. 



2. Large, coarsely granular cells (fig. 15, g. c), in which I 

 have not been able to discover nuclei. 



3. Cells which stain faintly (fig. 15, b. c), and have lar°-e, 

 deeply-staining nuclei. The cell protoplasm is very finely 

 granular. 



Thin laminae of this connective tissue (fig. 12, Ps.„) pass 

 across the atrial cavity into masses of similar tissue (tic. 12 

 P*.,), which surround the alimentary canal, thus acting as 

 mesenteries. 



Large, irregularly-shaped processes of this connective tissue 



