HISTORICAL SKETCH. XIX 



with the suprabranchial organs of vertebrates. The remarkable structure of the 

 olfactory organs in Limulus was also described for the first time. 



In 1894 it was shown that the exoskeleton consisted of a complicated, and 

 for an invertebrate, a very remarkable system of chitenous trabeculae resembling 

 a primitive form of dermal bone. 



In 1896 was published a paper on the "Variations in the Development of 

 Limulus." It was undertaken in the hope that it might throw some light on the 

 normal development, or give some indications of the kind of variations that have 

 led to the higher types. 



In 1899 and 'oo, in cooperation with Mr. Redenbaugh and Miss Hazen, a de- 

 scription of the peripheral nervous system, endocranium, and coxal glands was 

 published. The work was begun with the purpose of furnishing a detailed 

 account of the various systems of organs in arachnids as a basis for further com- 

 parisons with vertebrates. 



In 1901 advantage was taken of a six months leave of absence from college 

 duties to study the principal collections of ostracoderms in European museums. 

 It was rarely possible to make use of such collections for anything more than a 

 superficial examination. An effort was therefore made to obtain material that 

 could be sectioned, or used in any manner that seemed desirable, in order to get 

 at the anatomical structure. A valuable collection of Tremataspis and Thy- 

 estes was obtained in the island of Oesel in the Baltic Sea, and a few cephalaspids 

 and pteraspids were obtained by gift and purchase in England. In the next four 

 or five years an effort was made to obtain ostracoderms in the vicinity of Dal- 

 housie, N. B., Canada, at first with little success. Finally, I obtained a very large 

 number of specimens in a beautiful state of preservation, from which it was 

 possible to work out the anatomy in great detail. The structure of the eyes, jaws, 

 and internal organs afforded a striking confirmation of our conclusion that the 

 ostracoderms form a new class of animals standing between the vertebrates and 

 arachnids. 



In 1888, '89, and '90, Gaskell published his first papers on the Origin of the 

 Central Nervous System of Vertebrates. The basis of his theory was that "the 

 central nervous system of a crustacean ancestor had grown round and enclosed 

 the alimentary canal." " The ventricles of the brain were the old cephalic stomach 

 and the canalis centralis of the spinal cord, the long straight intestine which led 

 originally to the anus." The vertebrate develops a new heart, alimentary canal, 

 and other organs to take the place of those enclosed in the central nervous system. 



In its conception and mode of analysis of the conditions in the vertebrates and 

 arthropods, this theory is entirely different from, and wholly irreconcilable with my 

 own. In my judgment, the foundations on which it is built are totally wrong. 

 The fundamental error, which is inextricably interwoven in all his conclusions, 

 making a detailed criticism of them unnecessary, is the assumption that the neural 

 surface of an arthropod is the same as the haemal surface of a vertebrate. In this 

 confusion of opposite surfaces, which is like starting on a voyage of discovery with 



