ON THE ORIGIN OF VERTEBRATES FROM ARACHNIDS. 351 



coiled muscle-fibres and gland-like cells. As this remarkable 

 body represents an isolated and specialised segment of the 

 spinal artery, I have called it the merochord. In the adult 

 it is a rather compact ball of tissue lying between the brain 

 and the posterior portion of the cartilaginous sternum, or 

 endocranium (Fig. 12, mc). 



In Lepidoptera (Cecropia) the '^lemmatochord " is 

 derived in part from the neurolemma of the persistent median 

 nerve, and in part from the neurolemma of the lateral cords; 

 but in some parts of the thorax of Cecropia the lemmatochord 

 is entirely derived from the neurolemma of the median nerve, 

 the nerve itself having disappeared. The median nerve in 

 some cases runs in the centre of a spinal artery (S. Selvatico, 

 'Zool. Anz.,' Aug., 1887, p. 562). 



These facts show that the median furrow is a much more 

 important organ, morphologically, than has been supposed. 

 It is certain that the interganglionic portions of the furrow, or 

 at any rate something that cannot be distinguished morpho- 

 logically from them, may give rise to an extraordinary variety 

 of structures — to the furcse (in the thorax of Acilius), the 

 median nerve (in the abdomen), the lemmatochord, or at 

 least a portion of it, in Lepidoptera, and to the spinal artery 

 of Scorpio. Practically it makes little difference whether we 

 regard the spinal artery of Scorpio as derived from the 

 median nerve itself, its sheath, or a lemmatochord-like organ 

 with traces of the median nerve in the centre. The important 

 fact remains that, in a great many Arthropods there is a 

 median cord, which in position and general character bears 

 such an extraordinary resemblance to the notochord of Verte- 

 brates, that the burden of proof lies with those who deny that 

 the two cords are of the same nature. If it is urged that in 

 Arthropods the median cord arises from the ectoderm while 

 the notochord arises from the endoderm, we may safely answer 

 that there is nothing in the embryology of Vertebrates to show 

 to what germ layer the notochord belongs. It is never con- 

 tinuous with functional endoderm ; there is no evidence that 

 it ever exercised, itself, any alimentary functions; it is never 



