XIH PHYLUM CHORDATA 129 



circular canals. The kidney is a mesonephros, the ureter an archi- 

 nephric 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, 

 belonging to the genera Petromyzon, Mordacia, Geotria, and 

 Ichthyomyzon. 



ORDER 2. MYXIXOIDEI. 



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 Slime-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 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 of the skin and from 

 the segmentally arranged mucous glands. 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 

 flesh as it goes. 



The buccal funnel is edged with tentacles (Fig. 761) ; there is 

 a single median tooth above the oral aperture, and two rows 

 of smaller teeth on the tongue. The papillae beneath the cone- 

 like horny teeth bear a still closer superficial resemblance to rudi- 

 ments (or vestiges) of true calcified teeth than_is the case in the 



VOL. II K 



