114 



Comparative Morphology <>f Chordates 



CYCLDSTOME 



Fig. 318. Skeleton of a cyclostome, Petromyzon. (After Dean. Courtesy, Neal and 

 Rand: "Chordate Anatomy," Philadelphia, The Blakiston Company.) 



arches embedded in the connective-tissue sheath surrounding the 

 spinal cord. This sheath is continuous with the similar investment of 

 the notochord (Figs. 318, 319). The cartilaginous cranium is incomplete 

 and the visceral skeleton consists of an irregular cartilaginous "bran- 

 chial basket" (Fig. 319) instead of definite arches. The gill-chambers 

 are numerous. Some species of Bdellostorna have as many as 14 pairs. 

 Petromyzon has seven pairs. 



Cyclostomes have no paired fins. There are no scales in the highly 

 mucous skin. The nasal chamber in the adult is single and median, 

 but it originates by fusion of two embryonic nasal sacs. The single 

 external naris is median and dorsal. Some species are hermaphroditic. 



Petromyzon undergoes metamorphosis during its development. Its 

 larval stage is known as ammocoetes (Fig. 316B). This larva has a 



lc him mv so /</ m gc bb pc 



Fig. 319. Ventral and lateral views of the skull of lamprey (Petromyzon marinus). 

 (ad) anterior dorsal cartilage; (bb) branchial basket; (en) cornual cartilage; (gc) 

 gill-cleft; (lc) labial (annular) cartilage; (Idm) lateral distal mandibular; (Ig) 

 lingual cartilage; (mv) median ventral cartilage; (na) neural arch; (nc) nasal cap- 

 sule; (ncfi) notochord; (oc) otic capsule; (on) optic nerve; (pc) pericardial cartilage: 

 (pd) posterior dorsal cartilage; (pi) posterior lateral cartilage; (so) subocular arch. 

 (After Parker. Courtesy, Kingsley: "Comparative Anatomy of Vertebrates," 

 Philadelphia, The Blakiston Company.) 



