xni 



PHYLUM CHORDATA 



133 



to the middle line, the Lamprey differs from Amphioxus and 

 resembles all the rest of the Craniata. The blastopore does not 

 close up, but is converted into the anus, so that there is no procto- 

 dseum. The dorsal lip of the blastopore, very prominent 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 stomodseal invagination. 



The young is hatched as a peculiar larval form called Ammoccetes 

 (Fig. 815), which differs from the adult in several respects. 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. The 



buccal cavity is separated A , f ' *jf\ ul ^^~*& 

 from the pharynx by a I Yy * 



velum. A ciliated peri- 

 pharyngeal 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 

 thus proved to be a 



s 



special development of a 

 structure corresponding to 

 the endostyle of Amphioxus 

 (p. 46) 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. 



br.J 



Fio. 815. Petromyzon fluviatilis. Head of larva. 

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

 branchial aperture ; eye, eye ; 1. I. lower lip ; na. ap. 

 nostril; u. 1. 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 

 interior of the buccal funnel and on the large " tongue." Paired fins 

 are absent. There is no exoskeleton ; the skin is glandular. The 

 vertebral column consists of a persistent notochord with a fibrous 

 neural tube, in which rudimentary neural arches may be developed. 

 The skull is largely or wholly roofed by membrane, and there is 

 an extensive development of labial cartilages. The segments of 

 the post-auditory region of the head are more distinct than in the 

 rest of the Craniata. The enteric canal is straight, and there is no 



