HARD WICKE 'S S CIENCE - G O SSIP. 



243 



of cup-shaped cavities, in which the Polypes of the 

 united colony are lodged. In the Actinozoa we find 

 hard structures of great variety and interest, and which 

 have even borne an important part in the formation of 

 our earth ; I mean those commonly known as Corals. 

 Corals are of two kinds, sclerobasic and sclerodermic. 

 The difference between these I will try to explain. 

 Let us take the common Sea-anemone as a type of the 

 class, although it has no hard parts. We find that 

 this animal has a cylindrical body with a disk-shaped 

 foot below, by which it can adhere to the rocks and 

 move from place to place, and above, a crown of ten- 

 tacles, in the centre of which is the mouth. If we 

 took a slice across the body, we should find in the 

 interior a number of radiating partitions like those 

 seen when a poppy-head is cut across. Now if we 

 had a number of sea-anemones united together by the 

 edges of the foot, so as to form a hollow cylinder, the 

 inner surface of which was formed by the feet, and 

 that the feet then secreted calcareous matter, so as to 

 fill up the interior of this tube and convert it into a 

 hard axis, we should get a coral like the red and black 

 corals of which ornaments are made. If, however, the 

 tissues of the body wall and of the radiating partitions 

 were converted into hard stony tissue, we should get 

 a coral of the other or sclerodermic class. Corals 

 of the second class are sometimes simple, as many 

 of the cup corals, sometimes compound, like the brain 

 coral ; corals of the first kind are always formed by 

 compound animals, and they may be distinguished 

 from the others by not showing any cup with 

 radiating partitions. 



In the subkingdom Annuloida, the Scoleclda are 

 soft-bodied animals, many of which inhabit the bodies 

 of other animals. They are almost devoid of any 

 hard parts, although one kind of tapeworm has, as I 

 mentioned, a circle of hooks, _and the wheel animalcules 

 have a rather complicated set of horny jaws. 



In the Echinodermata, on the other hand, the hard 

 parts form a prominent and important feature. The 

 sea-urchin, for instance, has a globular shell composed 

 of hundreds of plates, which are arranged in ten double 

 alternately-dissimilar rows, reaching nearly from pole 

 to pole. At the apex of the shell are inserted small 

 plates which are perforated for the eyes, genital 

 apertures, and anus ; at the base is a gap, closed in by 

 membrane, in the centi-e of which is the mouth armed 

 with five teeth worked by the apparatus of which I have 

 before spoken. The plates of five of the double rows 

 are perforated with numerous holes for the passage of 

 the tubular feet. The surface is studded with tubercles, 

 on which are jointed movable spines : in some urchins, 

 as Cidaris, the spines are veiy large in proportion, and 

 are attached by a ball-and-socket joint, which reminds 

 one very strongly, in its arrangements of ligaments, of 

 the human hip-joint. There are also " pedicellariae," 

 very singular little organs with three snapping jaws, 

 tlie use of which is unknown. The shell of the Sea- 

 urchin grows by addition to the edges of the plates ; 



hence it does not need to be cast off like that of the 

 Lobster. The Star-fishes resemble the Sea-urchins, but 

 their plates are less closely attached together and the 

 spines are smaller. In the Sea-cucumbers there is no 

 distinct shell, but the plates of the Sea-urchins are re- 

 presented by scattered granules or spicules, often of 

 remarkable shapes ; thus in Myriotrochus they are 

 wheel-shaped, in Synapta like anchors jointed to per» 

 forated plates. The shells of the Echinoderms are 

 composed of carbonate of lime in the form of earthy 

 granules, which in the spines are arranged in a beauti- 

 ful radiating pattern with concentric zones like a section 

 of an exogenous tree. In the fossil state, however, the 

 hard tissues of the Echinoderms always break with 

 an oblique ciystalline fracture. 



In the subkingdom Annulosa, the material usually 

 made ixse of for the construction of the hard organs is 

 chitine, a substance resembling horn, but devoid of 

 any structure, and with little effect on polarized light. 

 Chitine, although a nitrogenous substance, is one of the 

 most indestructible of organic matters. Caustic alkalies, 

 prolonged boiling, and acids unless concentrated, have 

 no effect upon it ; strong sulphuric acid dissolves it 

 without charring. In the Annulosa the skeleton is- 

 external and jointed, and in all but the Annelids, as 

 the Leech, provided with jointed limbs. The seg- 

 mentation, as might be supposed, is more or less 

 marked, in proportion to the hardness of the skeleton j 

 tlius in the soft bodies of the Leech and Spider it is 

 much less conspicuous than in the hard armour of the 

 Lobster : sometimes, however, as in the Crab, several 

 segments are fused together into a shell or carapace. 



The Hermit-crabs, which have a soft unprotected 

 body, seek a protection by taking lodgings in the vacant 

 shell of some mollusk. In one order of Annelids, 

 the Tubicola, to which the Serpula belongs, the 

 integument is soft and smooth, but has the power of 

 secreting calcareous matter, which forms a fixed tube 

 or sheath, into which the animal can retire for safety. 



The Caddisvvomi, which is the larva of an insect 

 allied to the Dragon-flies, forms for itself a somewhat 

 similar but movable case by cementing together grains, 

 of sand, shells, and other small particles of matter. 



In the larger Crustacea, as the Crab and Lobster, the 

 shell is hardened with carbonate of lime. The shell 

 has a radiating fibrous texture, with contour-markings 

 indicating the layers of growth. As the shell of 

 the Crustacea forms a complete rigid case, in order to 

 allow of the growth of the animal, it has to be cast 

 off from time to time, and a new one formed. Prior 

 to the casting off of the old shell, a store of material 

 for the new one is laid up in the form of deposits of 

 carbonate of lime in various parts of the body. The 

 tendons are also calcareous. 



In Insects the external skeleton is chitinous, and 



is modified in different species to form implements. 



of various kinds, of wonderful elegance of form and 



exquisite adaptation to use. 



{To be continued.') 



M 2 



