ON THE ORIGIN OF VERTEBRATES FROM ARACHNIDS. 361 



head end to form a compact cephalothoracic shield or cranial 

 buckler — the post-cranial segments being in some cases pro- 

 bably soft and disconnected, — the shape of the swimming 

 appendages, and the probable method of locomotion. And, 

 moreover, since the concrescence of vagus segments and their 

 union with the true thorax is shown in the adult Scorpion and 

 also in Trilobites and Merostomata by the cervical suture, and 

 since a similar suture is seen in Pterichthys and Bothriolepis, 

 we may infer that the last two forms also possess vagus seg- 

 ments, and that their swimming appendages are homologous 

 with the combs of Scorpio and with the swimming legs of 

 Pterygotus. But as Pterichthys is also fish-like, its swimming 

 appendages are probably homologous with pectoral fins, and 

 consequently the combs of Scorpio are also homologous with 

 pectoral fins. 



In support of this view we may add (1) that the fundamental 

 structure of the pectens of Scorpio and the pectoral fins of 

 embryo Selacians is the same, for the framework of both con- 

 sists of a longitudinal bar, attached by its median end to the 

 body, which gives off" at right angles a series of rays. (2) 

 In Scorpio the pectens, which develop differently from any 

 other Arthropod appendages, first appear as an enormously 

 long transverse ridge, divided into a series of small lobes (PI. 

 XXIV, fig. 4). The important point is that the lateral end 

 is gradually separated more and more from the body, until 

 finally the whole appendage is only supported by a slender 

 stalk at its neural extremity. This method of development 

 agrees with the well-known development of the pectoral fins of 

 Selacians, and the result is a long basal bar attached at one 

 end only, and which may thus be swung forwards or backwards 

 as on a pivot, or even rotated on its long axis. (3) They 

 probably belong to the same segments, i.e. to the vagus. That 

 the pectoral fins arose very far forward is certain; their 

 probable origin from the vagus region is shown by the fact 

 that in Protopterus (Wiedersbeim) they receive nerves from 

 the vagus, and also by the fact that the remarkable longitudinal 

 muscles uniting the pectoral arch with the cranium are derived 



