No. I.J THE EMBRYOLOGY OF CREPIDULA. \\\ 



and some distance below the surface. The cells of the optic 

 cup which lie farthest to the right and left are the clear lens 

 cells. The cells at the bottom of the cup contain a black 

 pigment which is laid down at their inner ends. 



(d) TJie Cerebral Commissure. Figs, yd, ^^-82, g6. — The 

 commissure between the two ganglia is formed by an outgrowth 

 of elongated cells from the ganglia themselves. These out- 

 growths meet at the apical organ, forming a V-shaped structure, 

 the apical organ lying at the apex of the V. Later the two 

 limbs of the V fuse farther and farther away from the organ, 

 forming a Y, and finally a T. The bar of the T is the cerebral 

 commissure, and its stem represents the fused processes which 

 run from the middle of the commissure to the apical organ. 

 The fused character of this process is clearly seen in all the 

 later stages, where its double nature is plainly visible ; each half 

 is composed of only a single row of elongated fusiform cells. 

 Still later, with the degeneration of the apical organ, the stem 

 of the T disappears completely, leaving only the commissure. 



Similar strands of cells are found in other trochophore lar- 

 vae, e.g., in Teredo ('80) and Eupomatus ('86), according to 

 Hatschek, but they are said to be muscles, and not nerves. 

 The fact that in Crepidula these strands of cells arise from the 

 cerebral ganglia, and form, in part, the cerebral commissure, is 

 sufficient to prove that they are not muscles. I have, besides, 

 carefully studied the living embryos with regard to this point, 

 and have never seen any evidence of contraction in these cell 

 strands. I am therefore convinced that they are nervous struc- 

 tures, and am consequently inclined to assign to the apical 

 organ, to which these cell strands run, a sensory function. 



(e) The Cerebro-pedal Cotmectives. Figs, yd, 80-82, gy, 10^. 

 — A process of cells, similar to that which forms the cerebral 

 commissure, grows out from the ventral side of each cerebral 

 ganglion, and extends on each side of the oesophagus into 

 the foot, where it comes into close contact with the ectoderm 

 at the sides of the foot. This is the cerebro-pedal connective. 

 It is formed before there are any pedal ganglia, and it is 

 possible that those structures arise from cells which have 

 come down from the cerebral ganglia with the connective cells. 



