DISTINCTIVE CHARACTERS OF ANIMALS. 



177 



smaller ones, and each of which smaller ones again divides into two others ; 

 the same kind of division and subdivision being continued to a vast extent, 

 and every ray regularly decreasing in size, till at length the ramifications 

 amount to many thousands, forming a beautiful net-work spread over the 

 water. The colour of the worm varies : being sometimes pale, sometimes 

 reddish-white, sometimes brown. 



The only other genus I shall mention under this order is the echinus, sea- 

 urchin, or hedgehog : its species are very numerous, and .of a grest multi- 

 plicity of forms, globular, oval, shield-like, and heart-shaped. Many of them 

 appear to have long since become extinct, and are only .to be found in a state 

 of petrifaction. The surrounding spines form an admirable coat of mail when 

 perfect ; but they are generally broken off from the shell when it is picked 

 up empty on our own coasts. 



The THIRD ORDER of the Linnasan class of worms are called testacea or 

 TESTACEOUS ; and comprise those that are surrounded with a shelly or testa- 

 ceous covering. They are of three kinds ; those possessing a single shell, 

 of whatever form or kind, and hence denominated univalves ; those possessing 

 two shells, which are called bivalves or conchs ; and those possessing more 

 than two shells, which are in consequence named multivalves. 



The UNIVALVES, or single-valved, are the most numerous, and exhibit the 

 greatest variety of forms. For the most part they are regularly or irregularly 

 spiral : among the most common of them may be mentioned the helix or 

 snail-genus ; the patella or limpet ; and the turbo or wreath-genus, of which 

 the periwinkle is a species ; the animal in all which is a limax or slug. 

 Among the more curious are, the raurex or purple-shell so highly valued by 

 the ancients for the exquisite dye it is capable of producing ; the volute or 

 mitre, including those fine polished spiral shells, without lips or perforation, 

 which so often ornam'ent our chimney-pieces, sometimes embellished with 

 dots, and at other times with bands of colours of various hues ; the strombus, 

 comprising the larger shells appropriated to the same purpose, spiral like the 

 volute, but with a large expanding lip spreading into a groove on the left side, 

 and often still farther projecting into lobes or claws, the back frequently 

 covered with large warts or tubercles, in some species called coromant's 

 foot ; in all which, the animal or inhabitant is still a limax or slug ; and the 

 nautilus and argonauta, the pearl-nautilus and paper-nautilus ; the first of 

 which is lined with a layer of a most beautiful pearly gloss, and in the East 

 is manufactured into drinking-cups ; and the second of which is remarkable 

 for its exquisite lightness, and the rumour common to most countries of its 

 having given to mankind the first idea of sailing. In reality, it sails itself, 

 and with exquisite dexterity ; and to this end the animal that is usually found 

 inhabiting the shell, and which, till of late, was supposed to be a four-armed 

 cuttle-fish, though now regarded as an ocythoe, by Dr. Leach named o. Cran- 

 chii, in memory of the indefatigable, but unfortunate, Cranch of the British 

 Museum,* as soon as it has risen to the surface, erects two of its arms to a 

 considerable height and throws out a thin membrane between them, thus pro- 

 ducing a natural sail ; while the oars or rudder are formed by the other two 

 arms being thrown over the shell into the water, by which ingenious con- 

 trivance, or rather instinctive device, the paper-nautilus sails along with con- 

 siderable rapidity. M. Cuvier has separated the nautilus from the rest though 

 distinctly a univalve ; and, as we have already noticed, has united it with 

 the cuttle-fish, under an order of molluscs, which he calls cephalopoda. The 

 ordinal name for the others is with him gasteropoda, as most of them crawl 

 on their bellies, and carry the shell over them as a shield. They have a dis- 

 tinct and moveable head, by which they essentially differ from our next order, 

 which are without a distinct head of any kind. The two sexes are united in 

 the same individual, but require a reciprocal union for breeding. 



The BivALVED or two-shelled testaceous worms, the acephala or headless 

 of Cuvier, are best explained by referring you to the oyster and the muscle 



« Series I. Lecture xi p. 118. 



M 



