CHAPTER XXVIII 



THE CYCLOSTOMES, OR LAMPREYS 



HE Lampreys. Passing upward from the lancelets and 

 setting aside the descending series of Tunicates, we 

 have a long step indeed to the next class of fish-like 

 vertebrates. During the period this great gap represents in 

 time we have the development of brain, skull, heart, and other 

 differentiated organs replacing the simple structures found in 

 the lancelet. 



The presence of brain without limbs and without coat-of- 

 mail distinguishes the class of Cydostomes, or lampreys (/c-f/c-Aos, 

 round; <rro/ua, mouth). This group is also known as Marsipo- 

 branchi (papo-iTtiov, pouch ; fipdyxos, gill) ; Dermopteri (deppa, 

 skin; nrepov, fin); and Myzontes (^v^aaa, to suck). It includes 

 the forms known as lampreys, slime-eels, and hagfishes. 



Structure of the Lamprey. Comparing a Cyclostome with a 

 lancelet we may see many evidences of specialization in struc- 

 ture. The Cyclostome has a distinct head with a cranium 

 formed of a continuous body of cartilage modified to contain a 

 fish-like brain, a cartilaginous skeleton of which the cranium 

 is evidently a differentiated part. The vertebrae are undeveloped, 

 the notochord being surrounded by its membranes, without 

 bony or cartilaginous segments. The gills have the form of 

 fixed sacs, six to fourteen in number, on each side, arranged 

 in a cartilaginous structure known as "branchial basket" (fig. 

 2890), the elements of which are not clearly homologous with the 

 gill-arches of the true fishes. Fish-like eyes are developed on the 

 sides of the head. There is a median nostril associated with 

 a pituitary pouch, which pierces the skull floor. An ear-capsule 

 is developed. The brain is composed of paired ganglia in 

 general appearance resembling the brain of the true fish, but 



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