The name Acraniata (without skull) is often used for the lower Chordates taken collectively, and it is sometimes applied to the lancelets alone. It refers to those chordate forms which have no skull nor brain, as distinguished from the Craniota, or forms with a distinct brain having a bony or cartilaginous capsule for its protection.
Origin of Lancelets.—It is doubtless true, as Dr. Willey suggests, that the Vertebrates became separated from their worm-like ancestry through "the concentration of the central nervous system along the dorsal side of the body and its conversion into a hollow tube." Besides this trait two others are common to all of them, the presence of the gill-slits and that of the notochord. The gill-slits may have served primarily to relieve the stomach of water, as in the lowest forms they enter directly into the body-cavity. The primitive function of the notochord is still far from clear, but its ultimate use of its structures in affording protection and in furnishing a fulcrum for the muscles and limbs is of the greatest importance in the processes of life.
Fig. 289a.—Gill-basket of Lamprey.
[CHAPTER XXVIII]
THE CYCLOSTOMES, OR LAMPREYS
The 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 Cyclostomes, or lampreys (κυκλός, round; στόμα, mouth). This group is also known as Marsipobranchi (μαρσιπίον, pouch; βράγχος, gill); Dermopteri (δέρμα, skin; πτερόν, fin); and Myzontes (μυζάω, to suck). It includes the forms known as lampreys, slime-eels, and hagfishes.