2 PA TTEN AND REDENBA UGH. [Vol. XVI. 



and other questions of a purely theoretical nature. This mode 

 of treatment, therefore, is open to the constant danger of con- 

 fusing fact and theory, of destroying the simplicity and direct- 

 ness of the purely descriptive parts, and of detracting somewhat 

 from their permanent value. Moreover, comparisons instituted 

 for the purpose of indicating broad homologies fail to carry 

 conviction when standing alone, as they would have to do if 

 each system of organs were treated separately. In such com- 

 plicated problems as the origin of vertebrates, resemblances 

 between several systems of organs and the corresponding parts 

 in the hypothetical ancestors must be treated together, in order 

 to show how one comparison harmonizes with and supports the 

 other. It seemed best, therefore, to publish each descriptive 

 part as soon as completed, with only an occasional attempt 

 to point out the relations of certain organs in Liniidus to the 

 corresponding structures in vertebrates. At some later period 

 we hope to combine our results into an organic whole. 



While there may be differences of opinion as to the probable 

 value of such an undertaking, there can be no doubt that if 

 it leads to a fairly complete account of one species, that alone 

 will be a sufficient return for the labor, and will give the work 

 a value that cannot be derived from fragmentary accounts of 

 different, even though closely related, forms. Moreover, this 

 mode of treatment has obvious advantages, in that long famil- 

 iarity with a given form enables the investigator to avoid 

 the repetition of much preliminary work, and thus obtain his 

 results more rapidly. 



The senior author has for a long time regarded the endo- 

 cranium, the branchial cartilages, and the segmental cartilages 

 of the spinal cord of arachnids as forerunners of the corre- 

 sponding cartilages in vertebrates, and he began to study their 

 anatomy, histology, and development some ten years ago. A 

 preliminary statement as to the structure of the endocranium 

 in scorpions and Linmhis was given in his paper on the " Origin 

 of Vertebrates from Arachnids " ('89). Since then, from time 

 to time, many suggestive details have been gathered ; but as no 

 immediate opportunity was likely to present itself to get this 

 material into proper shape for publication, it was decided to turn 



