No. I.] GERM-LAYERS IN CLEPSIIVE. 159 



4. Primary Sense-organs of the Lip. 



At the time of hatching, long before the eyes and their serial 

 homologues, the segmental sense-organs, appear, two pairs of 

 sense-bulbs are found, symmetrically placed on the surface 

 that is to form the margin of the lip. These organs arise as 

 bulb-like thickenings of the epidermis. Figs. 12 and 13 repre- 

 sent successive sections through the anterior pair of bulbs (ysb'), 

 in the direction shown by the arrows 12-12 and 13-13 in Fig. 

 29. The fifth section behind that seen in Fig. 13 hits the 

 posterior pair of bulbs {sb"^, passing above the super-oesopha- 

 geal ganglia, as indicated by the arrow 14-14 in Fig. 29. 

 These sense-bulbs are nearer together than those of the anterior 

 pair, and they are a little depressed, as if there were a slight 

 infolding. The scanty material at my disposal did not permit 

 me to trace the history of these organs farther. I have since 

 given some time to the study of the development of the eyes 

 and sense-organs of the lip, in much later stages, and I have 

 found that all the sense-organs of Clepsine arise in the same 

 manner as these two pairs of bulbs. As the entire oesophageal 

 nerve-collar is already formed, there is absolutely no ground 

 for supposing that these bulbs are rudiments of the nervous 

 system. The basis for the super-cesophageal ganglia (^siip. oe. 

 g.) is present as a distinct body of cells in the stage of Fig. 20, 

 long before the appearance of the primary sense-bulbs. Even 

 as early as the stage of Fig. 3, I find between the epidermal 

 layer of the cephalic lobe and the primary entoderm cells a 

 layer of cells which I regard as the basis of both the neural 

 and the mesodermic elements of the head. The precise origin 

 of this layer I have not thus far determined. 



5. The Nephridia. 



In describing the origin of the nerve-chain I have called at- 

 tention to the nephridial rows of cells, two of which are found 

 in each germ-band, lying between the median (neural) and the 

 lateral rows (Figs. 2, 6, 7, 8, 9, 11, nph)^ and forming one stra- 

 tum with them. These relations are shown in Diag. 9. 



In C. parasita the nephridial rows are remarkably distinct, 

 owing to the contrast in color between them and the rows 

 by which they are bounded. This contrast, due to the coarse 



