446 



CYCLOSTOMATA 



[CH. 



longitudinal dorsal and ventral bars, and connecting cross-pieces pass- 

 ing between the sacs which give off branches encircling their outer 

 openings (Fig. 220). The whole of the branchial basket, as it is 

 called, is a development of the dermis and has probably nothing to 

 do with the visceral arches of the Craniata, as will be shown 

 later. Since however bars corresponding to true visceral arches are 

 found in Hemichorda and Cephalochorda, it is almost certain that 

 their absence in Cyclostomata is a secondary phenomenon, and 

 indeed, as we shall see immediately, remnants of true visceral 

 arches are possibly represented by certain cartilages connected with 

 the skull. The reason for their disappearance is a difference in the 



FIG. 220. A, dorsal, B, lateral, and C, ventral view of the skull of Petromyzon 

 marinus x 1 (after Parker). 



1. Horny teeth. 2. Annular cartilage. 3. Anterior labial cartilage. 



4. Posterior labial cartilage. 5. Nasal capsule. 6. Auditory capsule. 

 7. Dorsal portion of trabeculae. 8. Lateral distal labial cartilage. 



9. 'Lingual cartilage. 10. Branchial basket. 11. Cartilaginous cup 

 supporting pericardium. 12. Sheath of notochord. 13. Anterior 

 neural arches fused together. 



method of expanding the branchial sacs from that which obtains in 

 all other Craniata. For whereas in these latter the branchial sacs 

 are expanded by the spreading out of the visceral arches like the 

 ribs of an umbrella and are contracted when these arches fall 

 together, in Cyclostomata each sac has its own radiating and 

 circular muscles which enable it to contract and expand like a 

 heart, and the rhythm can be reversed since water can and does 

 enter as well as leave the gill-slits by the external pores when the 

 animal has its mouth applied' to a victim. 



The commencement of the true alimentary canal is marked, as 

 in Amphioxus, by a velum. What corresponds to the byper- 

 pharyngeal groove in that animal is in many species of Cyclostomata 



