ON THE ORIGIN OF VERTEBRATES FROM ARACHNIDS. 349 



the thickening is carried inwards ; when the process is once 

 started we get between the inner and outer ends of the oeso- 

 phagus all stages in the separation of the nerve from the ecto- 

 derm. The proximal ends of the nerves are united in the 

 middle of the fourth neuromere, and then terminate in a pair of 

 large ganglia. The latter arise not from the cesophageal walls, 

 but from an invagination of the median surface ectoderm, 

 and so close to the neuromere that there is no distinct boun- 

 dary between them (Figs. 7 and 10, g. ce., and Fig. 2, sp. n.). 

 I will also add that the unpaired frontal ganglion of insects 

 arises as an evagination of the oesophageal wall. Thus we 

 would naturally expect that if the infundibulum represents an 

 Arachnidan oesophagus, it would contain a large portion of 

 nervous tissue, and be directly continuous, through the rudi- 

 ments of the stomodeal ganglia, with the brain. (3) The 

 dorsal organ of some Arthropods is derived from the em- 

 bryonic membranes, and is finally invaginated into the yolk 

 and absorbed. In some Crustacea a dorsal organ is retained 

 throughout life, and is often used as a sucking or adhesive disc 

 of attachment. If an animal attached itself by such an organ 

 to the soft tissues of another, and if the inner portion of the 

 disc were absorbed by the yolk, a way would be opened for 

 the alimentary canal to communicate with the exterior by a 

 new sucking mouth, which would lie in about the same place 

 as the embryonic mouth of Vertebrates. In Dytiscus larvae, 

 where the points of the sickle-like mandibles are perforated 

 and serve as mouths, the old one becomes temporarily closed 

 and functionless. 



In Scorpio I have found nothing like a dorsal organ, but 

 in Limulus embryos there is a great mass of loosely connected 

 cells near the anterior dorsal surface of the yolk, or about 

 where the dorsal organ in other forms is present. In most of 

 my sections this portion was cut out, so that I know little 

 about it. It is not impossible that it represents a rudimentary 

 dorsal organ, all the stages in the formation of embryonic 

 membranes, &c., being omitted. 



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