248 Afi2Jects of the Bodjj in Vertebrates and Arthropods. 



that of Vertebrates. The crust, the segments, and tlie appen- 

 dages especially have nothing in common with Vertebrates, 

 though the functions are in a degree the same. Tlie origin 

 and homologies of the sensory organs are ah initio different. 

 For example, the eyes of Arthropods are not truly homolo- 

 gous with those of Vertebrates ; the cornea is simply a 

 number of epithelial cells, while in Vertebrates the eye 

 externally is an ingrowth of the epiblast. As the wings and 

 legs of insects and organs of hearing and of smell are not 

 the homologues of the parts which function as such in Verte- 

 brates, so we are not inclined to regard the heart and nervous 

 system of Arthropods as truly homologous with the corre- 

 sponding organs of Vertebrates. If there is such a funda- 

 mental difference in the two types as regards the relations of 

 the viscera to the body-walls, and if this relation is common 

 to all Arthropods and the Annulata, we shall have to go 

 back to the hypothetical common ancestors of the Tunicates 

 and Vertebrates on the one hand, and of the Annulata and 

 Arthropoda on the other, for the means of comparison. It is 

 not impossible that in animals allied to the Planarian or 

 Nemertean worms, whose nervous system consists of a pair of 

 dorsal ganglia, with two or more pairs of nerves passing back- 

 ward, that the common origin of the prochordate nervous 

 system and liiat peculiar to Annelids and Arthropods may yet 

 be discovered. 



So also the resemblance of the brain, dorsally situated, of 

 the Cephalopods, enclosed as it is in an imperfect cartilaginous 

 capsule, is interesting ; but the relations are those of analogy 

 or adaptation, and not of affinity. The Mollusks, the Annelids, 

 the Arthropods, and the Vertebrates appear to be highly 

 specialized branches, and where there appear at first siglit to 

 be direct cross-homologies, so to speak, between them, these 

 are rather independent structures, the result of adaptation 

 rather than of du'tct descent. Examples of sucli, we believe, 

 arc the eye, the brain, and the heart of the Cephalojjods. 



The unity of organization in the animal world is seen 

 rather in the homolugy of the cellular structure and in the 

 common origin of all from unicellular forms, and among the 

 Metazoa in the identity of the morula and gastrula cjndi- 

 tions, or at least the germ-layers; and as regar^ls the ne.'vous 

 system, in its origin in the epiblast, rather than in any special 

 parts or organs of such highly elaborated and si)ecialized 

 types as are represented by the lobster, or butterfly, or fish. 



The dispute between Cuvier and St.-lJilaire and their 

 followers was in part metaphysical. The old-time problems 

 in transcendental anatomy, such as comparing a lobster to a 



