August 1, 1891.] 



KNOWLEDGE 



153 



to that great group of animals known as Zoophytes, which 

 iuchides the Polypes, Sea-Anemones, and Corals. They 

 afford an example of the so-called " alternation of genera- 

 tions " ; being themselves developed by the division of a 

 fixed polype into a number of saucer-like sections, which 

 become free and swim away, and in turn lay eggs, again 

 developing into fixed Polypes, like the original parent. 



Our next illustration is taken from the Crustaceans, in 

 which the Lobsters and some of the Crabs are expert 

 swimmers. In the Lobster and Cray-Fishes, where the tail 

 is long, and furnished with five hinged and paddle-like 

 plates, the most rapid motions in the water are effected by 

 suddenly bending the tail beneath the body, and thus 

 dri^'ing the creature forcibly backwards by the recoU 

 of the water. Prawns and Shrimps have a similar mode 

 of swimming ; but those Crabs which, like the " Fiddlers," 

 are free swimmers have the terminal joints of the fifth 

 pair of legs (and sometimes also those of the three next 

 pairs) developed into flat paddles fringed with hairs. 

 These claws are thus quite diflerent from the pointed claws 

 of the common Shore-Crab. Since Crabs and Lobsters 

 breathe by means of gills, they may be safely regarded as 

 primitive swimmers ; those species which, like the Land- 

 Crab, are terrestrial having acquired this habit, and thus 

 having to put up with the inconvenience of keeping their 

 gills constantly moist. We cannot take leave of the Crus- 

 taceans without mentioning the Barnacles, as represented 

 by the common Acorn -Barnacle covering the rocks on our 

 coasts, and the Stalked-Barnacle which is more commonly 

 found on the bottoms of ships. In their young state these 

 curious creatures are free-swimming Crustaceans, but after 

 a time, becoming tired of a roving life, fix themselves on 

 their backs by the front of their heads to some solid object, 

 and then develop their well-known shells ; the feather-like 

 fan which protrudes from the aperture of these shells being 

 the greatly modified legs, now acting as feelers for the 

 purpose of capturing food. What induced the strange 

 belief that the Stalked-Barnacles underwent a further 

 metamorphosis to appear as Beruacle-Geese, passes 

 ordinary comprehension. 



Passing on to the Arachnids (Spiders and Scorpions) 

 and Insects, we find that these creatures, whether aquatic 

 or terrestrial, breathe atmospheric air by means of a 

 system of tubes known as tracheal and we are accordingly 

 led to conclude that such of them as are adapted to an 

 aquatic life have acquired this habit. This is especially 

 well shown by the instance of the Water-Spider, which, 

 while agreeing in structure with other spiders, has the 

 limbs fringed and somewhat flattened for swimming, and 

 is in the habit, when diving in the water, of carrying 

 down with it a bubble of air clinging to the hairs of the 

 abdomen. 



Among the Insects, the larvae of many groups in which 

 the perfect animals inhabit the air, such as the Dragon- 

 Flies, May-Flies, and Gnats, are aquatic. Whereas, 

 however, the larvie of the two former groups are not 

 swimmers, and, therefore, do not come witliin the scope of 

 the present article, those of the Cmats are endowed in 

 great perfection with the power of swimming. With their 

 large heads and their tapering bodies, these ugly larva' are 

 probably famiUar to most of us, swimming about in ponds 

 and tanks with great agility by a sudden jerking motion 

 of the body, or at intervals suspending themselves head 

 downwards at the surface of the water for the purpose of 

 breathing through a tube situated in the tail. Other 

 insects are aquatic in the adult state. Some of the 

 commonest British examples are the Water- Scorpion 

 (.Yc/xi) and Water-Boatman {Xotonecta), both belonging 

 to the order Ehynehota. These swim by means of the 



hind legs, which are, however, scarcely fringed in the 

 former, although markedly so in the latter. The Water- 

 Scorpion has two tail-like organs at the end of the body, 

 which, when put in opposition, form a tube through which 

 the creature can breathe without coming quite to the 

 surface. The Water-Boatman, as its scientific name 

 implies, has the curious habit of swimming on its back ; 

 when at rest for the purpose of taking in a fresh supply of 

 air the long hind legs are extended nearly at right angles 

 to the body, and thus recall a boatman resting on his 

 oars. * 



The remaining aquatic insects are the Water- Beetles 

 (Coleoptera), of which there are several families, in all of 

 which both the larvfe and adults are aquatic, although the 

 pup;E are quiescent and lie hidden in holes in the ground. 

 The Water-Beetles are easily recognized by their oval, fiat, 

 and boat-like form ; some of the species attaining a large 

 size. Most of them swim entirely by the aid of their hind 

 legs, which are greatly enlarged, flattened, and fringed. In 

 the curious little " Whirligig " Beetles {(Ti/rinus), which 

 are so often seen performing their mazy evolutions on the 

 surfaces of ponds and rivers, the reverse of this arrangement 

 obtains, the front pair of legs being enormously elongated, 

 and the second and third ]3airs very short and paddle-like. 

 The forward motion of these beetles is produced by these 

 short paddles, while the curves are formed by the long fore 

 limbs, which are darted out first from one side and then 

 from the other, so as to change the direction of the body. 



The last Invertebrate group we have to mention is the 

 large one of the Molluscs, or Shell-fish. Here by far the 

 greater number of species are aquatic, and breathe by gills, 

 so that we may regard those which are swimmers as being 

 primitively so. Although the adults of the Bivalve Mol- 

 luscs are either fixed to some solid substance (Oysters), or 

 are merely capable of leaping or turning (Cockles and 

 Fan-shells), yet in their young state aU these JIoUuscs 

 are free swimmers, young oysters being pronded with 

 swimming organs composed of delicate hairs. It thus 

 seems probable that these locomotive habits have been 

 transmitted to the young bivalves fi-om originally free- 

 swimming ancestors. 



The ordinary Sea Snails (Gastropods), in which the 

 adult creeps on solid surfaces by means of its greatly 

 expanded " foot," are also free swimmers when first 

 hatched, the swimming being efi'ected by means of 

 vigorous flappings of a pair of fins attached near the 

 head. A similar structure and habits have been retained 

 in the adult by the Pteropods, those small translucent 

 Molluscs, of pelagic habits, which are so abundant in some 

 of the northern seas, and afford a considerable proportion 

 of the food of certain species of whales. A well-known 

 writer states that •' Multitudes of these little things may 

 now and then be seen on the surface of the water, flutter- 

 ing with their wiugs and glittering in the sunshine, to be 

 compared with nothing more aptly than a congregation of 

 the more dressy of the Bombyx Moths." 



Although not a true swimmer, the weU-known Violet 

 Snail ([(uitliiiiii) is able to float on the surface of the ocean, 

 either by expandmg its " foot," or by developing at certain 

 seasons a peculiar membranous raft-like structure, the cells 

 of which are filled with air, and beneath which the eggs 

 are carried. 



A totally difi'erent mode of progression through the 

 water is adopted by that group of Molluscs technically 

 known as Ceplialopods, in which the head is surroimded by 

 a circle of long prehensile arms, provided with adhesive 

 suckers. This group comprises the existing Cuttle Fishe.s, 

 Squids, Argonauts, and Nautili, as well as the extinct 

 Ammonites (Fig. 2), and a host of other fossil forms. In 



