W. K. BEOOKS ON THE GENUS SALPA. 91 



As two accounts are therefore necessary, I shall first describe the 

 history of the chain-salpa as it would be if there were no secondary 

 changes, and I shall illustrate this first account by imaginary diagrams. 

 I shall also refer from time to time to figures of actual sections which 

 show the points which are referred to, as this will gradually familiarize 

 the reader with the secondary changes, although it will be as well for 

 him to refrain from any attempt to analyze these secondary changes 

 until they are taken up and discussed in due course. 



We left the stolon at the stage which is shown in Plate XXXIV, 

 Fig. 1, in which it consists of a tube of ectoderm, a, colored violet in the 

 figures ; inclosing a nerve tube, I, also colored violet, and running along 

 the middle line of the upper surface of the stolon ; an endodermal tube, d', 

 colored red ; a right perithoracic tube, /*, and a left one, g, both colored 

 green ; a string of ova, n, colored orange, inclosed in a tubular follicle, w, 

 colored blue ; and a lower ha3mal tube, i, and an upper onB, j, colored 

 yellow. All these structures must be pictured as running along the 

 stolon from its base to the point where it begins to strobilize into salpa3. 

 Three longitudinal sections, with the same colors and letters, are shown 

 in Figs. 4, 5 and 6. Fig. 4, which is near the bottom of the stolon, cuts 

 the ectoderm, a, the lower ha3mal tube, /, and the string of eggs, n, in its 

 follicle, m. Fig. 5 cuts first, on the right the ectoderm, a, then the right 

 perithoracic tube below its lumen, then a number of scattered mesoderm 

 cells, left uncolored, between the perithoracic tube and the area of 

 thickened endodermal epithelium on the right side of the endodermal 

 tube, d', then the ordinary flattened endodermal epithelium on the left, 

 then, in the upper part of the figure, the lower blood tube, i, with its 

 endothelium, colored blue, and in the lower part the layer of cellulose, 

 32, which fills up all the unoccupied spaces outside the haemal tubes, as is 

 shown in Fig. 3, and finally, on the left the section again cuts the ecto- 

 derm, which, in the section which was drawn, shows by its undulating 

 outline the first trace which I have found of the segmentation of the 

 stolon into salpae. Fig. 6 is a little higher up, and it passes through the 

 lumen of the right perithoracic tube, h, and through mesoderm cells 

 between it and the ectoderm, as well as between it and the thickened 

 endoderm on the right of the endodermal tube. The section passes 

 through the thickened endoderm on the left side of the endodermal tube, 

 but it passes below the left perithoracic tube. 



I have not figured a longitudinal section through the nerve tube, 

 Fig. 1, 1, at this stage, as it would be shown as a simple continuous tube. 



