188 SPECIAL PHYSIOLOGY. 



wards in the water, more easy to be maintained ; for without it, the specific 

 gravity of the upper or dorsal half of a fish is greater than that of the under 

 or abdominal half, owing to the presence of the vertebral column in the former, 

 a difference which would be easily diminished by a minute horizontal column 

 of air placed in the upper half. The adaptability of different fishes for the act 

 of swimming, differs exceedingly according to their form ; the swiftest swim- 

 mers are those in which the form is rather elongated (herring, salmon, shark) ; 

 the heterocercal fishes, which have the vertebral column prolonged to the ex- 

 tremity of the upper caudal fin (sharks), are swifter swimmers than the ordi- 

 nary or homocercal fishes, in which the cleft caudal fin extends beyond the 

 vertebral column. Many fishes swim very rapidly ; the salmon is said to 

 travel from twenty to twenty-five miles in an hour. Certain globular forms 

 of fish (diodon, sun-fish) either move sideways slowly, or turn over and over 

 in the water. 



Fishes which swim in the third general mode mentioned above, viz., by 

 lateral undulations, effect this object on the same principles as have been al- 

 ready explained in regard to the water-snakes ; but they swim submerged in- 

 stead of on the surface, and invariably have the advantage of the extension of 

 their lateral area by a continuous marginal dorsal and ventral fin. In the 

 flat fishes (sole, turbot), the flapping of the body and the undulatory or wrig- 

 gling movement of the marginal fins, produce their effect by striking the water 

 obliquely backwards, but in an upward and downward, instead of a horizontal 

 direction ; the reaction of the water upon them, takes place therefore along 

 two converging lines, from above and below, instead of from the sides. In 

 the true flat-fish which have no air-bladder (sole, turbot), the flat form of the 

 body is owing to an extension of the neural and haemal spines, the fish swim- 

 ming with one side, which is generally white, downwards, and the other, 

 which is brown, upwards. In the skates, rays, and torpedoes, the flat form is 

 owing to the extraordinary development of the anterior or pectoral fins ; these 

 spread out horizontally, and are provided with innumerable digits which sup- 

 port the soft parts, and thus form large lateral fins, which, in addition to the 

 tail, are used in swimming. 



In the Molluscous animals inhabiting the water, swimming is performed 

 either by the movement of their long arms acting as paddles (cephalopods), or 

 by special little lateral wing-like paddles (pteropods), or by fin-like expansions 

 of the foot, and vertically flattened tail (heteropods), or by aid of the movable 

 respiratory organs or gills (certain marine gasteropods). Of the lamelli- 

 branchiate forms, some are fixed, like the oyster, others are attached by a 

 byssus, as the mussel and pinna ; some float in the water, and others, like the 

 cockle, jump through the water from the bottom, by aid of their long curved 

 fleshy foot ; some bury themselves in sand, whilst others bore into rocks or 

 timber. The free moving Molluscoids mostly float, as the tunicata. 



In the Annulose creatures, many, such as the Crustaceans, move by means 

 of the paddle-like action of their numerous limbs, some of these (lobsters and 

 shrimps) also jumping or propelling themselves backwards in the water by 

 rapid flexure of the tail, which for that purpose is fitted with expanded ter- 

 minal appendages ; others of this subkingdom, as the water-beetles, use their 

 limbs as oars (notonecton) ; others move by the action of multitudes of lateral 

 setse attached to each successive segment (aphrodite, sea-mouse) ; others swim 

 by undulatory movements of the body, either by lateral (vermes), or by verti- 

 cal undulations (leech) ; others again, in the larval stage of their existence, 

 propel themselves by ejecting water from a receptacle in their body, backwards 

 from their caudal extremity, a movement characterized, from its resemblance 

 to the action of a syringe, as syringograde. Of the swimming Annuloids, in 

 certain echinodermata, the swimming motion is sometimes pinnigrade, or per- 

 formed by movable pinnate arms, as in the crinoidea ; the rotiferous animal- 

 cules move in the water by means of their cilia ; the marine worm-like scole- 

 cida move by an undulatory action of their bodies. 



In the free moving Ccelenterata, one form of movement in the water, often 

 named pulmoyrcufa, is performed by rhythmical contractions, which occur once 

 in about eighteen seconds, of the entire umbrella-like disc of the animal (Me- 

 dusee), and which might be compared to the pulsations of the heart, or to the 



