Class 2. Pisces. Order 1. Cyclostomi. 38S 



before a cliange occiors, are very diflei-ent in form from tlie adiilt ; homy teeth 

 are absent, the eyes are very small, and the gill-sacs open directly into the mouth. 

 They live in mnd. 



2. The H a g - f i s h (Myxine) has rudimentaiy eyes ; the mouth is surrounded 

 by tactile tentacles ; the gill-sacs (six on each side) are long tubes expanded in 

 the middle, each opening direct into the pharynx, whilst the outer regions of 

 each side unite, to open by a common apex-ture some way back ; though in an 

 allied foreign form, Bdellostoma, they open separately. Hag-fish, of which 

 M. glutinosa is very common in N. European seas, and reaches as much as 

 30 o/m. in length, bore into dead (and living P) Fish; they secrete enormous 

 masses of mucus. 



Order 2. SelacMi. 



The skeleton consists entirely of cartilage, vf-hioh may, however, 

 be partially calcified ; bone is altogether wanting. A conus 

 arteriosus is present, and a spiral valve in the gut. There are 

 five, rarely six or seven, gill-clefts on each side; often a spiracle,, 

 but no operculum, excepting in Chimffira. There is no swim- 

 bladder. The whole surface of the skin is often covered with teeth. 

 In the fins, which cannot be folded together, there are horny rays. 

 The mouth is on the ventral side of the head. Parts of the pelvic 

 fins iu the male serve as copulatory apparatus. Eggs very large.. 

 Almost exclusively marine. 



1. The Sharks (Sq-ualidx) are animals of the ordinary piscine form,, 

 genei-ally elongate and somewhat circular iu section. The skin is usually thickly 

 covered with small teeth. Along the edge of the jaw there are, as a rule, one oi' 

 two rows of teeth, usually triangular in form, and replaced by others from the 

 mucous membrane within the jaws : definitely heterooercal. Of the numerous forms 

 the following may be specially mentioned : the Common Spiny Dog-fish 

 (Acanihias vulgaris), 1 m. long, with a spine (a strongly developed placoid scale) 

 anteriorly on each of the dorsal fins ; anal fin absent ; viviparous : found in the 

 North Sea and Baltic: the Dog-fish (Scyllium cowicMZa), somewhat smaller ; 

 oviparous, egg-capsule quadrangular, and attached to Algse by long tendi'il-like 

 appendages from the comers; common on the coasts of Britain: the Blue 

 Shark {Carchwrias glaucus), the voracious man-eating form, 3 m. or 4 m. long, 

 ooctu-ring in the MediteiTanean, abundant in the Tropics: the Hammer - 

 headed Shark {Sphyrna), with each side of the head drawn out into a longer 

 or shorter process, at the end of which is the eye ; one species in the Mediterranean : 

 the Greenland Shark (Scymnus horealis), which reaches 8 m. in length, 

 is caught in great numbers for the sake of the fat liver ; on the coast of Iceland : 

 still larger (up to 12 m.) is the Giant Shark {Selacke maxima), in which 

 the external giU-clefts are very large sUts ; the eyes very small ; teeth small and 

 poorly developed ; the inner edge of the gill-bars, with a series of very long teeth 

 forming a fine comb, which acts as a straining apparatus, to retain the small. 

 Crustacea on which this giant feeds, after the manner of the Whalebone Whale. 



2. The Skates (Bajidse) are chiefly distinguished by the flattened form 

 of the head and body, by the thin, whip-like tail, which is often almost destitute 

 of fins, and by the enormous development of the peotoi-al fins, which arise like 

 horizontal plates from the sides of the body, so as to form a disc with it and the 

 head, and to relegate the giU-slits, over which they lie, to the ventral surface; the 

 eyes and spii-acle are on the dorsal side. Amongst other characters it must be 

 mentioned that the skin is usually naked over a greater or less extent ; that 



