654 COMPARATIVE ANATOMY 



cranial and spinal nerves, since they have diverse origins. In the head, 

 however, he recognizes in arachnids the twelve cranial nerves of verte- 

 brates from the olfactory to the hypoglossal. 



The peripheral nervous system of arachnids attains a condition similar 

 to that in vertebrates. The neural (ventral) nerves of arachnids like 

 the dorsal nerves of vertebrates, are ganglionated, and the hemal (dorsal) 

 nerves are non-ganglionated, like the ventral nerves of vertebrates. 



In order to compare arachnid and vertebrate hearts, Patten assumes 

 that the arachnid heart represents the ventricular portion of vertebrates, 

 and that the posterior portion of the pericardial cavity of arachnids 

 corresponds with the atrium and sinus venosus of vertebrates. 



But even Patten, who is so impressed by the resemblances of scorpions 

 and vertebrates, is compelled to admit that comparison of arachnid with 

 vertebrate circulation is difficult. He is, however, curiously reticent in 

 regard to the fact that the blood in limulus flows to the heart from the gills, 

 and not the reverse as in vertebrates. In limulus five veins bring blood 

 from the gills to the pericardial cavity. It is difficult to understand why 

 Patten should label the last of these, and only the last, the ductus Cuvieri. 

 According to Patten, the auditory organ of vertebrates is represented 

 in the limulus embryo by a large discoidal placode which lies opposite 

 the fourth pair of legs. There is, however, no evidence that this is 

 auditory, or even sensory, and it disappears early in development. 



Both parietal and lateral eyes of vertebrates are homologous with 

 those of arachnids. During the evolution of vertebrates from arachnids. 

 Patten assumes, there was a considerable period during which the lateral 

 eyes were adjusting themselves to their new position inside the brain 

 chamber, and when they were in functional abeyance. When the lateral 

 eyes again became functional, the parietal eye began to decrease in size 

 and efifectiveness. 



"The characteristic shape of the arthropod eye and the arrangement 

 of its retinal cells is retained in an exaggerated form in the vertebrate 

 retina, and affords us the only satisfactory explanation of its inversion, 

 its contour and mode of growth, its choroid fissure, its arrangement of 

 rod and cone cells, and its centrally located optic nerve. 



"The agreement between the olfactory organ of limulus and that of 

 vertebrates may be traced in respect to so many different characters that 

 the existence of a genetic relationship between the marine arachnids and 

 the vertebrates is placed beyond a reasonable doubt. Indeed, there is a 

 greater difference in respect to this organ, between limulus and other 

 invertebrates than there is between limulus and vertebrates." 



The olfactory apparatus arose in the higher arachnids through the 

 secondary modifications of pre-existing organs which had some other 

 function or meaning. 



