CHAPTER XIII 
THE CYCLOSTOMES, OR LAMPREYS 
A) 
HE Lampreys.—Passing upward from the lancelets and 
setting aside the descending series of Tunicates, we 
Ew} 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 Cyclostomes, or lampreys (xuKAos, 
round; oroya, mouth). This group is also known as Marsipo- 
branchi (uapoiniov, pouch; fpayyos, gill); Dermopteri (dépua, 
skin; zrepor, fin); and Myzontes (uvfaw@, 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. 
116), the elements cf 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 ‘s composed of paired ganglia in 
general appearance resembling the brain of the true fish, but 
167, 
