242 



HARD Wl CKE 'S S CIENCE - G OSSIP. 



Sea-urchin there are five pointed teeth arranged in a 

 circle round the mouth, and attached above to a com- 

 plicated mechanism of arches and levers, called 

 "Aristotle's Lantern." The Leech has three horny 

 serrated jaws, in shape like segments of a circular 

 saw, and which produce the well-known three-rayed 

 bite. In crustaceans and insects the masticatory 

 organs are modified limbs ; in insects they vary veiy 

 much in shape according to the nature of the food, 

 from the short, strong jaws of the Wasp to the long, 

 slender proboscis of the Moth. The lower or headless 

 mollusks have no teeth ; in the higher mollusks, as 

 the Snail, the dentition is very curious : there is a 

 band, the lingual ribbon, which is set with innumer- 

 able minute teeth, and which, being drawn backwards 

 and forwards over a cartilaginous pulley, rasps the 

 food. In the Limpet, this ribbon is nearly twice the 

 length of the entire body. As the teeth in front wear 

 away, their place is supplied by fresh ones from 

 behind. In vertebrates, we most frequently find the 

 jaws set with teeth : in birds, however, the jaw- 

 bones are covered with horn, and form a pointed 

 beak ; and this form is again met with in the Turtles, 

 and even among mollusks, as in the Cuttle-fish. In fish 

 and most reptiles the teeth are of a piece with the jaw- 

 bones, and grow in a continuous succession : as one 

 drops off, another comes forward to take its place. 

 In crocodiles and mammals, the teeth are implanted 

 in sockets in the jawbone. In mammalia, the shape, 

 number, and arrangement of the teeth vary greatly, 

 according to the nature of the food ; so that from the 

 teeth the habits of the animal to which they belong may 

 be deduced. Carnivorous animals have strong canine 

 teeth for holding their prey, and sharp -edged molars 

 for mincing up the flesh. Herbivorous animals have, 

 on the other hand, broad flat molars, adapted for 

 grinding, and the harder and softer tissues of the 

 tooth are so arranged that the unequal wear shall 

 preserve a rough surface, like that of a millstone. 



5th. As weapons of offence ; e.g. , the sharp fin- 

 spines of some fisheg, as the Stickleback ; the claws of 

 the carnivora ; the horns of the Rhinoceros and the 

 ruminant animals ; and the strongly-developed tusks 

 or canine teeth of many others. In venomous snakes, 

 certain of the teeth are channelled and furnished with 

 poison-glands at the base. In many animals which 

 ftght for the possession of the females, these weapons 

 of offence are only met with in the male sex, as the 

 horns of the stag, and the tusks of the boar and male 

 ape. This fact has furnished Mr. Darwin with a 

 strong argument in favour of the modifying operation 

 of "sexual selection." 



6th. They form part of the mechanism of many 

 special organs ; as those of the senses and voice. As 

 examples, I may quote the ossicles and otoliths of the 

 ear, the bony plates met with in the eyes of birds, and 

 still more strongly developed in those of the extinct 

 Ichthyosaurus, the spongy bones of the nose, the 

 liyoid bone which forms the fulcrum for the tongue. 



the cartilages of the larynx or organ of voice, and the 

 bony centre in the bullock's heart. 



I projjose briefly to run through the animal kingdom, 

 and note the different materials and mechanisms which 

 we find in the different classes. 



The simplest animals of all, Rhizopoda, consist 

 merely of homogeneous specks of animated jelly, of 

 which every part is capable of performing all the 

 functions of the animal. Some of them, as Amoeba, 

 have no hard parts at all ; others form minute calcareous 

 shells of the most varied and beautiful forms. The 

 Foraminifera are so called from the fact that in one 

 division of the order the shells are pierced by numerous 

 minute holes for the extension of the radiating 

 tentacles. In some Foraminifera the shell, however, is 

 not perforated. In the perforated species the shells 

 are often transparent, in the imperforate kinds they 

 are of porcelain-like texture, or covered with grains 

 of sand cemented together. The shells are sometimes 

 single-chambered ; more often many chambers, each 

 rather larger than its predecessors, are clustered 

 together, forming shells which frequently resemble a 

 nautilus in shape. The great variety of forms, 

 through apparently very complicated, ai-e produced by 

 variations in the shape, relative size, and relative posi- 

 tion of the chambers. 



The Polycistina possess shells of equal beauty with 

 those of the Foraminifera, but differing from them in 

 shape, and in being composed of silica instead of 

 carbonate of lime. 



Scarcely higher in the scale of animal life are the 

 Sponges, the possession of which indeed the Botanists 

 long disputed with the Zoologists. The Sponges con- 

 sist merely of a framework covered with a soft animal 

 jelly. The skeleton differs in nature in different 

 classes of Sponges : in some, as the sponge of commerce 

 and the little freshwater Spongilla, it is composed of 

 horny fibres mixed with flinty needles or spicules ; in 

 others it is calcareous, as many of the fossil forms ; 

 while in a third class, including deep-sea forms, as the 

 beautiful Venus's flower-basket (Euplectella), it is 

 wholly composed of interlaced siliceous spicules. 

 Spicules are a kind of hard structure met with in 

 animals of other orders, differing widely from the 

 Sponges : they are of very various fornis ; some needle- 

 shaped, others like a toasting-fork, or thorny stick, or 

 two wheels and an axle. Some are composed of 

 carbonate of lime, others of silica. They are embedded 

 in the soft flesh of the animal, and their use is com- 

 monly believed to be to give consistence and support 

 to the soft tissues ; but Mr. Wallace believes that they 

 also serve to render the animal uneatable, and thus 

 protect it from those creatures who would otherwise 

 devour it. 



Passing to the Coelenterata or Polypes, we find 

 very frequently in the Hydrozoa, of which Sertularia 

 pinnata, the zoophyte commonly found on oysters, 

 may be taken as an example, a horny branched poly- 

 pidom, or common skeleton, furnished with a number 



