38 CYCLOSTOMATA 



Craniata apertures for the 9th and 10th cranial nerves are 

 included in the occipital region of the skull, in the Cyclostomata 

 both these nerves pass out freely behind the auditory capsule, 

 beyond which the cranium does not extend backwards. There 

 is a considerable gap, in Petromyzon, between the auditory capsule 

 and the large compound first neural arch through which also 

 pass the combined ventral roots of the first three spinal nerves 

 (Fig- 25). 



Considerable controversy has taken place concerning the 

 character of the branchial basket. It is often alleged that it is 

 not homologous with the branchial arches of the Gnathostomes, 

 since it lies in too superficial a position with regard to the muscles, 

 nerves, and blood-vessels, and pharyngeal wall. For this reason 

 also it is sometimes compared to the extrabranchials of Elasmo- 

 branchs. This objection is, however, unconvincing, for, although 

 it is true that the ventral aorta lies internal to the basket, yet the 

 relation of the cartilage to the vagus and spinal nerves, to the 

 dorsal aorta, to the myotomes, and to some of the visceral muscles, 

 is quite similar to that of a branchial arch. The proximity of the 

 basket to the integument at the sides may be, to some extent, due 

 to the peculiar development of the large sac-like gill-pouches (Fig. 

 35). Moreover, in Myxinoids the relative position of the three 

 anterior deeply-set gill-bars is normal (Ayers and Jackson [25], 

 Allis [17]). The branchial arches in both Cyclostomes and Gnatho- 

 stomes are probably derived from the same structures (Dohrn [1 14], 

 Gaupp [152]). 



The myotomes stretch uniformly from head to tail, bent in 

 ^ shape (Fig. 28), and are not subdivided into dorsal and ventral 

 halves by a horizontal septum as in Gnathostomes (Maurer [296]). 

 They are interrupted by the row of gill-openings in the lampre}" 

 and are prolonged forward over the face above and below the eye. 



An elaborate system of large muscles works the rasping 

 ' tongue.' Since they are supplied by branches of the 5th 

 cranial nerve (Fig. 20), it may be concluded that they represent 

 the visceral muscles of the mandibular region, and that the 

 cartilages to which they are attached correspond to the lower part 

 of the mandibular arch (Meckel's cartilage), and perhaps to the 

 hyoid arch as well (Ayers and Jackson). Development supports 

 the view that the ' tongue ' cartilages correspond to the mandibular 

 arch (Stockard). 



Some of the more interesting points in the development of the 

 muscles and nerves of the Cyclostomes have already been dealt 

 with above (p. 5, etc.). The permanent separation of the ventral 

 from the dorsal roots of the spinal nerves in the Petromyzontia, 

 and their junction in Myxinoidea, is very difficult to explain ([341], 

 Figs. 18, 20, 22). On the whole, it seems probable that the fusion of 



