PHYLUM CHORDATA 



137 



cells arranged as a longitudinal strand— the mesoderm plates. 

 In the head-region (Fig. 804, B) a number of diverticula from the 

 archenteron — cmlomic divertimda — are given off into these strands : 

 in the trunk region (A) these are absent. The inner portion of the 

 mesoderm on each side becomes divided up into a series of meso- 

 dermal somites or protovertebrse, the lateral part remaining un- 

 divided and forming the lateral plate. In this restriction of 

 somite-formation to the part of the mesoderm immediately adjacent 

 to the middle line, the Lamprey differs from Amphioxus and re- 

 sembles all the rest of the Craniata. The blastopore does not close 

 up, but is converted into the anus, so that therp is no proctodEeum. 

 The dorsal lip of the 

 blastopore, very promin- 

 ent from the first, becomes 

 produced to give rise to 

 the rudiment of the tail 

 region. The mouth is 

 developed later than the 

 anus b}' the formation of a 

 stomodseal invagination. 



The young is hatched 

 as a peculiar larval form 

 called Ammocmtes (Fig. 

 805), which differs from 

 the adult in several re- 

 spects. The median fin 

 is continuous. There is 

 a semicircular, hood- 

 shaped upper lip (u. I.) 

 instead of the suctorial 

 buccal funnel of the adult, 

 and teeth are absent. A 

 ciliated peripharyngeal 

 groove encircles the pharynx in front and is continued backwards 

 on the ventral side as a median groove opening behind into 

 the thyroid gland, which is thus proved to be a special develop- 

 ment of a structure corresponding to the endostyle of Amphioxus 

 (p. 48) and the Tunicata. The eyes are rudimentary and hidden 

 beneath the skin ; the brain is of far greater proportional size than 

 in the adult ; and, as already mentioned, the gill-pouches open into 

 the pharynx in the normal manner. 



Fig. 805.— Fetromyzon fluTiatills. Head of larva. 

 A, from beneatli; B, from the side. br. 1, first 

 branchial aperture ; ei/e, eye ; I. l. lower lip ; na. ftp. 

 nostril ; «. I. upper lip. (After "W. K. Parker.) 



2. Distinctive Characters and Classification. 



The Cyclostomata are Craniata in which the mouth lies at the 

 bottom of a sucker-like buccal funnel, or in a depression edged with 

 tentacles, and there are no jaws. Horny teeth are borne on the 



