ON THE OKIGTN OF VERTEBRATES FROM ARACHNIDS. 357 



the tissue between segmental respiratory organs or lung-books, 

 and not modified appendages ; consequently they differ from 

 the true appendages, like the mandibular and hyoid arches, in 

 not containing somatic diverticula (Van Wyhe). (5) The che- 

 licerse are represented in Vertebrates by adhesive pre-oral 

 papillae; this is not so improbable as at first might appear, for 

 in many Crustacea the first antennae, which are probably 

 homologous with the chelicerae, aided by the secretion of a 

 sticky substance, serve as sucker-like organs of attachment. 



XI. Muscles. — Since longitudinal muscles serve to move 

 the segments on one another, the complete fusion of the first 

 thirteen segments to form the cephalothorax is, without doubt, 

 the cause of the disappearance of the dorsal and ventral longi- 

 tudinal muscles of Scorpio. 



In the Vertebrate head the dorsal and ventral longitudinal 

 muscles are also absent ; therefore we conclude that in the 

 ancestral Vertebrates the segments were immoveably united to 

 form a hard outer skeleton like the thoracic shield of Arthro- 

 pods ; the partial union of soft, flexible parts, as in an Annelid 

 thorax, would not explain the absence of these muscles. As 

 longitudinal muscles, when present, are very large, we can 

 thus account, in a measure, for the great difference between 

 the development of the head and trunk somites of Vertebrates 

 and Arachnids. 



A more detailed comparison shows further that, (1) in both 

 Scorpio and Vertebrates, there is very little mesoblastic 

 tissue, and no distinct somite at all in the fore-brain. It is 

 even possible that all the original fore-brain mesoblast has 

 disappeared, that which is now present being derived from a 

 forward growth from the post-oral segments. (2) The mid- and 

 hind-brain region in both Vertebrates and Arachnids contains 

 six mesoblastic somites, from which diverticula are formed 

 leading into the appendages (gill-arches), and giving rise to 

 muscles passing from these appendages to the cartilaginous 

 cranium. (3) In the accessory brain region of Scorpions and 

 in the vagus region of Vertebrates the mesoblastic somites dis- 



