Mil 



I'llVLUM CHORDATA 



137 



l.l A 



7 \ X^'^^^/^ 



br.i 



cells arranged as a lonoitudinal sti-and — the mesoderm })latos. 

 In the head-region (Fig. 804, 1>) a niunberof diverticula fioni the 

 arehenteron — nrlumic dicerticula — are given off into tliese strands : 

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

 mesoderm on each side becomes di\idc(l u]) into a series of meso- 

 dermal somites or protovertebrje, the lateral part remaining un- 

 divided and fcjrming the lateral plate. In this restriction of 

 somite-formation to the part of the mesoderm inmiediately adjacent 

 to the middle line, the Lampre}' 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 there is no proctodaeum. 

 The dorsal li[) 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 by the formation of a 

 stomodfeal invagination. 



The young is hatched 

 as a peculiar larval form 

 called Ammoemtes (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. 



Fio. S05.— Petromyzon fluviatilis. Head of larva. 

 A, from buiieatli ; }i, fruin tlie side, br, 1, first 

 branchial aperture ; e//e, eye ; /. I. lower lip ; lUt. dp. 

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



2. Distinctive Chaeacters and Classification. 



The Cyclostomata are Craniata in which the mouth lies at the 

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

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



