138 ZOOLOGY sect. 



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 

 cloaca. The respiratory organs are six to fourteen 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 

 semicircular canals. The kidney is a mesonephros, the ureter a 

 pronephric duct. The gonad is unpaired, and there are no 

 gonoducts, the genital products making their exit by genital pores. 

 The Class is divided into two Orders. 



Order 1. — Petromyzontes. 



Cyclostomata in which there is a well-developed dorsal fin and 

 a complete branchial basket; the pituitary sac terminates 

 posteriorly in a blind pouch ; the gills open into a respiratory 

 tube below the gullet. This order includes the Lampreys, 

 which belong to the genera Petromyzon, Mordacia, Geotria, and 

 IcMhyomyzon. 



Order 2. — Myxinoidei. 



Cyclostomata in which the dorsal fin is absent or feebly 

 developed ; the branchial basket is reduced to a vestige ; the 

 pituitary sac opens posteriorly into the mouth ; the gills open 

 into the pharynx in the normal manner. 



This order includes the Hags or Sliine-eels, belonging to the 

 genera Myxine and Bdellostoma. 



3. — Comparison of the Myxinoids with the Lamprey. 



The organisation of the Lampreys is so uniform that all that 

 will be necessary in the present section is to indicate the principal 

 points in v^■hich the Hags difl^er from them. 



Myxine is about the size of a fresh-water Lamprey — i.e. some 

 forty-five cm. long : Bdellostoma is fully a metre in length. Both 

 are remarkable for the immense quantities of slime they are 

 capable of exuding from the general surface and from the seg- 

 mentally arranged mucus-glands of the skin. It is said that two 



