656 COMPARATIVE ANATOMY 



The segmental taste organs and slime buds of the arachnids are the 

 forerunners of the special cutaneous organs of vertebrates, the taste organs 

 of the arachnids corresponding with the taste-buds of the vertebrates, 

 and the slime buds probably in part with the neuromasts of lateral line 

 organs. 



Nothing resembling the dermal skeleton of limulus occur in any other 

 invertebrate. But this skeleton resembles that of the ostracoderm, 

 Pteraspis, more than does the skeleton of any other known anim.al, 

 vertebrate or invertebrate. Since ostracoderms are vertebrates, the 

 dose affinity of limulus to vertebrates is thus attested. The ostracoderm 

 skeleton is a highly specialized one, produced by an exaggeration of the 

 type of skeleton seen in limulus. Patten, however, admits the possibility 

 that the exoskeleton of ostracoderms may be mesodermal and not, like 

 that of Hmulus, ectodermal. But in his opinion, very slight changes 

 would be necessary to convert the skeleton of limulus into that of verte- 

 brates. He is inclined to assume that the mesenchyme which forms 

 dermal bone in vertebrates is of ectodermal origin. The. fact that the 

 ectodermal part of the skeleton of limulus is chitinous while that of verte- 

 brates is enamel does not disturb Patten, since in his opinion they "have 

 essentially the same structure and mode of growth." 



Patten also finds a cartilaginous endocranium characteristic of the 

 arachnid-vertebrate stock. "The fully developed arachnid endocranium 

 is in every essential respect a duplicate of the primordial cranium of a 

 primitive vertebrate embryo." They agree, he says, in position, general 

 form, mesodermal origin and histological structure, absence of metamer- 

 ism, and in their planes of growth. 



The resemblance also holds for the branchial skeleton. In limulus, 

 as in vertebrates, the "branchial bars" are postrcranial, and may be 

 united by longitudinal bands. The gill-bars of both consist of muco- 

 cartilage and not, like the cranium, of fibro-cartilage. The pre-auditory 

 region is without gills and gill bars. 



The neural arches of limulus represent the initial stages in the forma- 

 tion of a vertebral column. They agree in general form and in the direc- 

 tion of their processes, with the neural arches of vertebrates. The 

 homologue in arachnids of the notochord of vertebrates is the bothroidal 

 cord or lemmatochord. Lemma tochord and notochord have according to 

 Patten a similar embryonic origin and similar relations to the nerve cord 

 in the adult. 



Objections to the Arachnid Hypothesis of Patten. It is customary 

 to object to the arachnid hypothesis on the ground that arachnids, such as 

 Hmulus, are highly specialized animals, while evolution proceeds by the 

 transformation of generalized forms. The very fact that limulus has 

 changed little, if at all, in miUions of years might seem to weaken the 



