308 THE ORIGIN OF VERTEBRATES 
surrounds the throat ventrally and laterally, and is the most bulky 
muscle in the suctorial apparatus, 
The velar or mandibular nerve supplies in Ammoccetes the 
muscles of the lower lip. In Petromyzon it supplies also the 
longitudinal muscles of the tongue. The tongue-cartilage first 
develops in the region of the median ventral tentacle, and there 
the longitudinal tongue-muscles first begin to develop, not from 
the rudimentary muscles in the tongue but from those in the 
lower lip region. 
In Ammoccetes the tentacular nerve supplies the rudimentary 
muscles in the tentacles and the muscles of the upper lip. The 
latter disappear entirely at transformation, and in Petromyzon the 
tentacular nerve supplies the circular, pharyngeal, and annular 
muscles, which are derived from the rudimentary tentacular 
muscles. 
For the convenience of my reader I append here a table showing 
my conception of the manner in which the endognathal and ecto- 
gnathal segments of the Paleostracan are represented in Ammoccetes. 
It shows well the uniform manner in which all the individual 
segmental factors have been fused together to represent the appear- 
ance of a single segment (van Wijhe’s first segment) in the case of 
the four endognathal segments, but have retained their individuality 
in the case of the ectognathal segment. 
on .|o 
eZ/ER A d . : Dorso- 
2a Se iil Appendage | Skeletal Pe gente Ceelomic Coxal 
S 8 E g : nerves. elements. nerves, |S¢smental) cavities. | glands. 
3 | Be | Eurypterid. | Ammoceetes muecles. 
i 1 Pitui- 
1 Ten- 1 Ten- Oculo-; Sup. | 1 Pre- tary 
3! | 4 Endo- | 4 Ten- tacular | tacular | ™0tor | inf. int.| mandi- body ; 
1 gnaths tacles to 4 bar to 4 | SUPply-| rectus | bular fusion of 
4 tentacles | tentacles |_'™8 = 4 obo Net) aes 4 coxal 
muscles | oblique| of 4 
5 glands. 
1 Troch- 
learis 
1 Ecto- 1 Tongue | 1 Tongue Sup. | 1Man- 
28 gnath 1 Tongue |“ nerve bar pe a Wy- oblique | dibular 
muscle 
