370 HOWARD AYERS 
BDELLOSTOMA 
Great as the structural differences between Amphioxus and 
Ammocoetes undoubtedly are, they are not in some respects 
greater than the structural differences between Ammocoetes 
and Bdellostoma. 
It is an accepted fact that Ammocoetes has progressed much 
further toward the condition of anatomical organization found in 
fishes higher in the scale of development than has Amphioxus. 
It is also true that Bdellostoma has reached a much higher state 
of organization than Ammocoetes and in general in the direction 
toward fishes reckoned as more advanced in structure, e.g., the 
Chimaeroids and sharks. It is generally held that the Myxi- 
noids are not jaw-bearing fishes, although the evidence is accumu- 
lating to prove that they are. Having presented evidence to 
show that the buccal skeleton and some associated structures 
in Amphioxus are homologous with the parts of the jaw apparatus 
of Ammocoetes, if we can now show that the heretofore puzzling 
jaw apparatus of Bdellostoma is built on the same plan and can 
establish the essential homology of its parts with these of Am- 
phioxus and Ammocoetes, we shall advance far in our effort 
to solve the jaw problem. 
The skeleton, mus¢éles, and nerves of Bdellostoma have been 
subjected to thorough investigation, but have not by any means 
yielded up all they have to tell. No complete description of 
these structures will be given here, but only such details as seem 
pertinent to the problem under consideration. 
The skeleton of Bdellostoma is both cartilaginous and mem- 
branous; many structures important for comparison with higher 
forms are still in the membranous stage of their development. 
The comparison of the parts which have become cartilaginous 
with the same membranous territories in Ammocoetes and Ani- 
phioxus is indeed instructive and the same may be said for carti- 
laginous parts in the higher fishes which remain membranous in 
Bdellostoma. In such comparison is a rich mine of morpho- 
logical data. 
For more detailed accounts of the anatomy of the skeleton, 
muscles, and nerves the reader should consult the works of 
