THE CEPHALIC APPENDAGES AND THE LATERAL FOLD. 



269 



it is clear that the paired fins of vertebrates cannot represent any arthropod ap- 

 pendages in front of the sixteenth pair. 



The clue to the whole problem of vertebrate and arthropod locomotor ap- 

 pendages will be found in the ostracoderms, where both cephalic appendages of 

 the arthropod type, and lateral folds of the trunk, like those in true vertebrates, 

 are present. 



In Bothriolepis oar-like cephalic appendages are present that clearly belong 

 to the anterior division of the head, for they lie immediately behind the oral 

 arches and in front of the gills. (Fig. 4, A, 247.) In Cephalaspis the large 

 paddle-shaped appendages are of the same general nature and lie in a similar posi- 

 tion. (Fig. 232, 234.) In Tremataspis portions of armored appendages have been 

 found similar to those in Bothriolepis, that fit into the most anterior of a series of 

 nine pairs of notches, or openings. The eight posterior pairs served either for 

 the attachment of smaller appendages, comparable with external gills, or as 



D \ 



FIG. 174 



FIG. i; 



FIG. 174. Embryos of the sturgeon; showing the rudimentary cephalic appendages of the oral arches. 



After Salensky. 



FIG. 175. Oral region of an adult Cyclostome (Bdellostoma) showing three pairs of rudimentary oral-arch appen- 

 dages. Compare with the oral arches of an amphibian embryo, Fig. 161. 



openings to respiratory chambers containing the internal gills. (Fig. 236.) In 

 Cyathaspis, Drepanaspis, Palaeaspis, and Pteraspis, there are indications of ex- 

 ternal cephalic appendages in the form of armored "oars" like those in Bothrio- 

 lepis, or in the form of naked tentacles. (Figs. 244-246.) 



The cephalic appendages of the ostracoderms no doubt represent the same 

 kind of organs as the external gills, the balancers, and the oral arch tentacles of 

 primitive vertebrate embryos, and all these structures probably represent various 

 modifications of the cephalothoracic appendages of the arachnids. 



B. The Fringing Processes and the Lateral Fold. In those ostracoderms that 

 are well enough preserved to show the shape of the body, the postcephalic portion 

 has a triangular outline in cross-section, with either a series of distinct separately 

 movable plates of peculiar structure on either margin, or a continuous membran- 

 ous fold, with or without, supporting specules or minute ossicles. 



In Bothriolepis the trunk was practically naked and was provided with two 

 narrow membranous folds projecting freely from the haemo-lateral margins. The 

 folds extended from the root of the trunk backward, uniting with each other on 



