THE ANCESTRY OF THE VERTEBRATES 65 1 



cavities resembling those of vertebrate embryos are found in limulus. 

 Limulus has seven prootic segments, and a similar number should be found 

 in vertebrates — all supplied by the trigeminal nerve. Of these seven 

 segments, the first is represented in vertebrates by Miss Piatt's anterior 

 somite, segments 2 to 5 by the pre-mandibular cavity, 6 and 7 by the 

 mandibular cavity. The chiasma of the trochlearis nerve is explained 

 by the fact that, in scorpions, the homologue of the superior oblique 

 muscle, alone of all the head muscles, crosses the mid-dorsal line to be 

 attached to the other side, carrying its nerve with it. 



The tentacles of ammocoetes are assumed to be remnants of the 

 prosomatic appendages of the paleostracan ancestor. The mucocartilage 

 head-shield of ammocoetes is considered to be the exact counterpart of the 

 head-shield of cephalaspis. 



Gaskell interprets the flabellum of limulus and the pectens of scorpions 

 as homologues of the auditory organ of vertebrates. The various endo- 

 crinal organs of vertebrates — pituitary, thymus, etc. — are assumed to 

 come from the coxal glands of Protostraca, or primitive arthropods, which 

 were the common ancestors of arachnids and Crustacea. 



Gaskell's speculations necessitate the invention of a notochord and a 

 new alimentary canal to replace the old one which formed the neural 

 tube. He imagines that among the Protostraca were forms somewhat 

 resembling trilobites but with annelid affinities, which, like Apus, possessed 

 a deep ventral groove extending from one end of the body to the other, 

 and also pleural fingers, as in many trilobites. This groove on the ventral 

 side of the body was converted into a tube, and so gave rise to the noto- 

 chord, while the appendages were still free and the pleurae had not met 

 to form a new ventral surface. (See Fig. 527.) 



Passing from this protostracan to a paleostracan stage, oral and 

 respiratory chambers were formed, not communicating with each other. 

 A ventral groove, however, connected the respiratory chamber and cloaca. 

 Finally, with the conversion of this groove into a tube to form an alimen- 

 tary canal, the opening of the oral into the respiratory chamber, and the 

 formation of an atrium by the ventralward growth of the pleural folds, 

 the vertebrate was completed. The pharynx was formed by the fusion 

 and concrescence of the crustacean legs, the spaces between the legs 

 becoming gill slits. Because of the digestive power of the epidermis, 

 Gaskell sees no difficulty in converting skin into a digestive canal. 



The suggestion that the notochord may have originated as an accessory 

 digestive tube accords with the evidence that it is formed in some verte- 

 brates in connexion with the alimentary canal. 



Gaskell asserts that since the germ-layer theory is discredited by 

 embryologists themselves, it may not properly be raised against his theory, 

 which ignores it. Gaskell attacks the germ-layer theory as fallacious. 



