DiSTINCTIVE CHARACTERS OF ANIMALS. 



189 



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 1 shall mention under this order, is the echinus, 

 sea-urchin, or hedge-hog : its species are very numerous, and of a great 

 multiplicity 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 Linnean class of worms are called testacea 

 or testaceous ; and coniprise those that are surrounded with a shelly or 

 testaceous 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 pos- 

 sessing more than two shells, which are in consequence named multi- 

 valves. 



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

 bit 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 murex or purple-shell s0 

 highly valued by the ancients for the exquisite dye it is capable of pro- 

 ducing ; the volute or mitre, including those fine polished spiral shells, 

 without lips or perforation, which so often ornament our chimney-pieces^ 

 sometimes embeUished 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 hke 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 tuber- 

 cles, 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 hghtness, and 

 the rumour common to most countries of its having given to mankind the 

 first idea of sailing. In reaUty, 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. Cranchii, in memory of the 

 indefatigable, but unfortunate branch 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 producing 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 contrivance, or 

 rather instinctive device, the paper-nautilus sails along with considerable 

 rapidity. M. Cuvier has separated the nautilus from the rest, though dis- 

 tinctly an univalve ; and, as we have already noticed, has united it with the 

 *cuttle-fish, under an order of MOLLusciE, which he calls cephalopoda. The 

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



See Series I. Lect. XI. p. 119. 



