128 



ZOOLOGY 



SECT. 



/.// 



has the form of a longitudinal keel devoid of a cavity ; the neuro- 



coele appears subsequently. According to some observations the 



ventral portion of the 

 keel gives rise to the 

 notochord, which is thus 

 an ectodermal, and not, 

 as usual, an endodermal 

 product. 



The young is hatched 

 as a peculiar larval 

 form called Arnmoccetes 

 (Fig. 760), which dif- 

 fers from the adult in 

 several respects. It has 

 a semicircular, hood- 

 shaped upper lip (u. I.) 

 instead of the suctorial 

 buccal funnel of the 

 adult : 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. 7(50. 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. 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, and has 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 enteric canal is straight, and there is no cloaca. 

 The respiratory organs are six or seven pairs of gill-pouches. 

 There is no conus arteriosus and no renal portal system. There 

 are distinct cerebral hemispheres, which may be either hollow or 

 solid : the cerebellum is very small. Each optic nerve passes 

 directly to the eye of its own side. The olfactory organ is single 

 and median, but is supplied by paired olfactory nerves ; it opens 

 into a large persistent pituitary sac which perforates the basis 

 cranii from above. The auditory organ has one or two semi- 



