134 ZOOLOGY SECT. 



cloaca. The respiratory organs are six to fourteen pairs of gill- 

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

 There are large olfactory lobes, which may be either hollow or 

 solid ; the cerebellum is very small. 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 

 Ichthyomyzon. 



ORDER 2. MYXINOIDEI. 



Cyclostomata in which the dorsal fin is absent or feebly 

 developed ; the branchial basket is reduced ; the pituitary sac 

 opens posteriorly into the mouth ; the gills open into the pharynx 

 in the normal manner. 



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

 genera Myxine, Paramyxine, 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 which the Hags differ 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 

 specimens of Myxine thrown into a bucket of water are capable 

 of gelatinising the whole with their secretion. The slime-glands 

 of Myxine contain peculiar " thread-cells " containing a much- 

 coiled thread which unwinds either before or after the discharge 

 of the cell from the gland. 



Myxine approaches most nearly to the condition of an internal 

 parasite of any Vertebrate ; it is said to attach itself to living 

 Fishes and gradually to bore its way into the ccelome, devouring 

 the flesh as it goes. 



