356 WILLIA^[ PATTEN. 



reduced, by parasitism or a variety of other causes, to trans- 

 verse ridges (like the pectens, for example, PI. XXIV, figs. 3 

 and 4 ; or like the thoracic and abdominal appendages of 

 Limulus), and the segmental tubes enlarged to great trans- 

 verse respiratory slits, like those in the abdomen of Scorpio, 

 then we should have a condition much like that of the gill- 

 slits and gill-arches of Vertebrates. Such an Arthropod 

 appendage would resemble a gill- arch (I) in being supplied 

 with a neural nerve ; (2) in containing an artery following the 

 nerve first mentioned ; (3) in possessing a great sense-organ, 

 from which a ganglion to the neural nerve arises ; (4) in the 

 origin of its muscles from a diverticulum of a mesoblastic 

 somite ; (5) they would agree approximately in number with 

 true gill-arches ; (6) they would agree with gill-arches in their 

 serial physiological differentiation, for in both cases the ante- 

 rior pairs are of great size, forked, and serve as mouth parts, 

 the posterior ones being associated with respiratory organs, and 

 showing a tendency to degenerate. 



Segmental respiratory sacs or tubes are eminently charac- 

 teristic of Arthropods ; and, as they are probably derived from 

 the outlets of nephridia, they represent just the kind of respira- 

 tory organs required, according to Dohrn's theory, in ancestral 

 Vertebrates. 



In Scorpio the wandering backward of vagus nerves to 

 abdominal lung-books is important, and shows that we may 

 not, without other evidence, assume that in Vertebrates the 

 true gill-arches agree in number with or belong to the same 

 segment as the nerves that supply them. Accepting Van 

 Wyhe's views as to the structure of the Vertebrate head, we 

 offer the following tentative conclusions : — (1) The chelae and 

 first, or perhaps first and second, walking legs of Scorpio cor- 

 respond to the mandibular and hyoid arches. (2) The remain- 

 ing two or three pairs of thoracic appendages and somites are 

 not present in Vertebrates. (3) The rudimentary vagus ap- 

 pendages of Scorpions and the corresponding somites, except 

 the muscles extending to the pectoral or pectinal arches, have 

 iu Vertebrates disappeared. (4) The true gill-arches represent 



