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THE AMEBIC AN NATURALIST [Vol. XL VI 



eies of this sort, wherever occurring, and unfortunately they 

 occur in many sections of the vast realm, I am ready at all 

 times to do battle. 



So far this review and commentary has been made entirely 

 from the standpoint of observed and observable facts dealt with 

 in the volume. Now the point of view must be shifted to the 

 theoretical side. 



Patten's central thesis, as is well known, is that vertebrates 

 have descended from arachnids. The "arachnid theory of the 

 origin of vertebrates," or, for short, the "arachnid theory," is 

 the phraseology used by the author. The theory was first defi- 

 nitely set forth in 1889, the title of the original publication being 

 "On the Origin of Vertebrates from Arachnids." A somewhat 

 abbreviated quotation of the author's outline of the theory will 

 be justifiable. He writes: 



This theory has formed the basis of all my subsequent work, and as 

 far as it went, is practically the same as the one presented here. In that 

 paper it was maintained that the vertebrates are descended from the 

 arachnid division of the arthropods, in which were included the typical 

 arachnids, the trilobites, and merostomes. The ostracoderms were re- 

 garded as a separate class, uniting the arachnids with the true verte- 

 brates. Limulus and the scorpion were the types most carefully studied, 

 because they were the nearest and most available living representatives 

 of the now extinct merostomes, or giant sea scorpions, that were re- 

 garded as the arachnids standing nearest to the ostracoderms. 



Other evidence and conclusions were as follows: (1) In the arachnids 

 a forebrain vesicle is formed by the same process of marginal over- 

 growth as in the vertebrates. . . . (2) The kidney-shaped compound eye 

 of arachnids has been transferred to the walls of the cerebral vesicle in 

 vertebrates, giving rise to the retina, which still shows traces of omma- 

 tidia in the arrangement of the rod-and-cone cells. ... (3) The arach- 

 nids have a cartilaginous endocranium similar in shape and location to 

 the primordial cranium of vertebrates. (4) They have an axial, sub- 

 neural rod comparable with the notochord. (5) In arachnids the brain 

 contains approximately the same number of neuromeres as in verte- 

 brates. . . . (6) The segmental sense organs (median and lateral eyes, 

 olfactory and auditory organs) are comparable with those in vertebrates- 

 The coxal sense organs are associated with special sensory nerves and 

 ganglia, comparable with the cranial dorsal-root nerves and ganglia 

 (suprabranchial sense organs) of verterates. (7) The basal arches of 

 the appendages are comparable with the oral and branchial visceral 

 arches in vertebrates. (8) The tendency toward concentration of 

 neuromeres has narrowed the passage way for the stomodeum and 

 modified the mode of life in the arachnids. This ultimately led to its 



