648 



SESUVIUM SHA R*K. 



havvkmoth). They are of comparatively rare oc- 

 currence. 



SESUVIUM (Linnaeus). A succulent genus of 

 plants bearing icosandrous flowers, and belonging 

 to Ficoideae. The species require to be placed in dry 

 porous soil, and to be sparingly watered. 



SETARIA (Beauvois). A genus belonging to 

 the GraminecE, found in hot as well as in cold coun- 

 tries. The S. Germanica is agricultural, and the 

 S. Italica is economical. 



SHADDOCK is the Citrus decumana of Linnaeus, 

 an East Indian tree of noble port, and bearing very 

 large fruit, the largest indeed of all the citron family. 

 Though far inferior to the orange for the table, or to 

 the lime or lemon for medical purposes, it is a most 

 useful fruit in those warm countries of which it is a 

 native. Their appearance when on the tree is magni- 

 ficent ; a richer scene cannot be imagined than a grove 

 of shaddocks, limes, mangos, guavas, &c., as may be 

 seen in many places on the coast of Coromandel. 

 The juice of the shaddock is fugitive, and therefore 

 the fruit cannot be kept any great length of time, 

 especially if laid on any hard substance. They are 

 usually slung on light nets in cool air. 



SHALLOT is the Allium Ascalonicum of Linnaeus, 

 a high-flavoured species of onion. Its offsets, by 

 which it increases itself, are planted in the autumn 

 or in early spring, and in the course of the summer a 

 numerous addition of new offsets are produced. 



SHARK (Squalus, or rather Squalidce, the shark 

 family). A numerous, powerful, and celebrated family 

 of cartilaginous fishes, with fixed gills, forming, along 

 with the rays (see RAIA), the second of the three fa- 

 milies into which Cuvier divides the cartilaginous 

 fishes, and the first of those that have the gills fixed, 

 and openings in the sides for discharging the water 

 when it is no longer fit for the purpose of breathing. 



SELACHII is the name which Cuvier gives to the 

 whole family, including the sharks and rays as sub- 

 families ; and as the sharks are the typical fishes, we 

 shall here mention the particulars in which they and 

 the rays differ from other fishes, and agree with each 

 other, and with the true fishes that have the gills 

 fixed. 



They have the ordinary bones of the jaws wholly 

 wanting, or at least only rudimental ; and the bones 

 of the palate, and post-rnandibular bones, are the only 

 ones furnished with teeth, and serve the purpose of 

 jaws very formidable, and indeed terrible ones in 

 some of the species. One bone unites the rudimen- 

 tal jaw bones on each side to the cranium ; and serves 

 for the tympanic, the temporal, the jugal, and the 

 preopercular bones in ordinary fishes. The os hi/aides, 

 or bone of the tongue, is attached to this single bone, 

 and bears branchiostizial arches for the gills, which, 

 however, are not so conspicuous to internal observa- 

 tion as those of the fishes that have the gills free. 

 There are also gill-arches, but all the three pieces 

 which usually form the skeleton of the gill-cover in 

 fishes, are wholly wanting. They have all pectoral 

 and ventral fins, the latter placed as far back as the 

 vent, indicating that they are fishes of forward mo- 

 tion, rather than ascenders and descenders of the 

 water. The habit is true to this part of their struc- 

 ture ; for almost the whole of them swim near the 

 surface, and all move rapidly through the water. 



The gills are not free at the posterior edge as in 

 other fishes, but fixed so that there is a separate cur- 

 rent between every two, and then either all pass off 



by separate openings, or are conveyed to a common 

 canal. .Besides the gill-arches, to which the inner 

 edges of the gills are attached, there are cartilaginous 

 arches embedded in the flesh opposite the exterior 

 edges, and it is by means of these, probably, that the 

 operation of breathing is carried on. This kind of 

 breathing apparatus may, at first consideration, ap- 

 pear to be less perfect than that of the fishes with 

 free gills ; but in reality it is the very reverse ; for 

 it is equally powerful and far more safe. Fishes 

 which have the gills free can be drowned or suffo- 

 cated, if the water enters their gills the wrong way ; 

 and thus they cannot remain in a current with their 

 heads downwards. It is not, of course, any thing 

 in the water that causes this ; for the water is 

 exactly the same, and the application of it must 

 tend to produce the same effect on the blood of the 

 fish, whether it enters the gills the one way or the 

 other. But when the gills are free at the one extre- 

 mity, a current of water entering the wrong way en- 

 tangles the fibres, presses them against the arches, 

 and of course stops the circulation of the blood in. 

 them. This cannot happen to those that have the 

 gills fixed ; and thus they can breathe the water with 

 equal safety, let it enter which way it will. This 

 gives to these fishes a power of endurance in the 

 water which other fishes do not possess ; and the 

 power is not given them in vain, for they make vigo- 

 rous use of it, not confining themselves to particular 

 latitudes, but careering over the widest oceans, and 

 braving the utmost turbulence of the waters, better 

 tempered to the perils of the deep than any other 

 inhabitants of the sea. 



There are other circumstances besides their secu- 

 rity in breathing which assist them in this respect. 

 Their cartilaginous bones make their bodies joint 

 all over ; so that they can yield every way to the 

 muscles, and are perfectly secure from fracture. 

 Their skins are also much firmer than those of the 

 bony fishes, and thus give firmer insertion to the 

 muscles. They are, in fact, a sort of intermediate 

 between animals articulated on the internal skeleton, 

 and on the external crust, and they thus partake of 

 many of the advantages of both kinds of organisa- 

 tion. Their muscles are also differently constructed, 

 as any one may see by comparing the flesh of a cod 

 or a mackarel with that of a skate. In the two for- 

 mer, as well as in every bony fish that could be ex- 

 amined, whether with spinous fins or not, the flesh is 

 always in flakes : whereas in the skate it is in long 

 fibres. These fishes, by having their motion rather in 

 the flexibility of the skeleton than in the articulation 

 of it, and in having the muscles thus formed, are 

 capable of moving upon any and every point of the 

 body as a centre, while the motions are as powerful 

 as they are varied. 



But there are still other circumstances connected 

 with the physiology of these fishes, which tend to 

 give them additional powers of remaining in the sea, 

 and being active and discursive in it, through the 

 greater part of the year. Their mode of reproduc- 

 tion does not consist in the bringing forward of a vast 

 roe and milt at one season, which demands a resort 

 to peculiar grounds, where these may be deposited 

 an operation which exhausts all the other fishes very 

 much. In some the young are hatched internally, 

 and in all they are so fecundated. Those, too, which 

 are excluded in the state of eggs, are each charged 

 with the rudiment of a perfect and independent life ; 



