NATURE NOTES

Evening Post, Volume CXI, Issue 79, 3 April 1926, Page 13

NATURE NOTES

MOLLUSCA

ANIMALS OF MANY FORMS

(By E. 11. D. Stidolph, E.A.0.U.)

•A 3chool teasher in •'Wellington ■ h«| asked tor information about mollu?c«j which Include? animal? tomriu&if-kno-,rn is •'"shell-fish," the snails; and 3iugs, and the'-simlas." Mollusc* "ari» tot., loid-bloodod »nhrials, <witb unieg-merited-and hmbiesf bodies, having'no;ia> ternal bony skeleton and iio cartilv gir.ous. tissues \excopi in the. cuttle,*, octopus, and nautilus) and. in the maji ontv r } ( cases -secreting a . 'shell- eithfle external at -lut.-nuii. Although differing greatly .in size 1 and form,''ce'riaJJt features arc commou to nearly all the families , With .'Use exception ;■ of this bivalves,, most of them possess a head provided .with- a 'mouth-containing thie characteristic tooth.'ribbon. In nearijr al. the families s foof or locomotive organ is present.' In the caso of ts« sn:ui,-it tasos the form of a sole-oV iv.Uich.the animal, creeps; iir theJ&il ■valvas, an axe-shapqd . pushing : instru* ment; jr. the slcphant tusk shells, a dig* gmg 01 boring instrument j or in the «iut» ties and nautilus, a series of "arms" about the mouth. The foot may, b« rudimentary or absent, as in the cis« with the sedentary molluscs, such as th# oyster; but with some active bivalves, bs, the winkle, the foot enables the nnimal to jump about. The shell, whjc> is a conspicuous feature in the Majority of molluscs is secreted by .the mantle, which, ie a fold of skin "which may either cover both sides of the body, ai in the bivalves, or part of the; baclf only, as in the univalves.. The •thret layers of the shell are secreted by. » serins of glands, along the.edge of th'# mantle,' of three kinds, corresponding to the three layers of the shell. The fhel{» take the most varied.forms. Some ar» double, as in the bivalvesj acme spiral, a? the whelk; elongated:and; tubular, as in the elephant tusk shells;' compof. Ed of a series of plates as in the chit* oris; concealed by the mantle as in th* slugs; ar quite internal, as the ao--oallo.d ""'boiie" n'f tliof ■-++ I—^- _■ ■■V'-niiV, they may .be smooth and i highly polished on the exterior, or covered witk ridgoß,; -knobs, and spires. Every known tint of colour, occurs in theif ornamentation.: ' ' . ■ . -,- ' , ■

. Molluscs enjoy a, wide, distribution. The majority are marine, but many inhabit fresh -tvater, and others are terrestrial. They occur from the shore; to ocean .depths of 2000 fathoms, anil fr.om the Tropics to the Arctic regions.. 'Jt is stated that there are over 50,000 living species, of which about 80 per cent, are included in the gastropods, and the bulk of the. balance in-theVbl-valvse. Most of the molluscs possess » well-developed circulatory system', centring in a heart. : Arteries and vein*, convey, the blood to and from the heart. It is aerated, in tl>e cafe nf niarinr species by gills.wind) extract oxygen from the. water. The tooth ribbon consists of a strip of chitin', eovored with numerous rows of teeth, ■'. which,. move* backwards :and forwards over a. pad. The food is thus rasped into fragment! before being swallowed and conveyod.to the crop. The five senses appear to::be provided. All molluscs are sensitive^to touch. Most of the species except tJi« bivalves, havo eyes in. their head;., in. many bivalves they are represented by sensitive ocelli arranged along the edges of the mantle. It is yery doubtful, however, if any can see much. The sense of smell is, well developed, but little is- known about:the^alfactory organs. The young aro always produced from eggs, which are usually hatched extenin to the body• of the parent; bint in the bivalves and some gastr.opodf, they' are retained within the body, or the shell of the parent till after'hatch* ing. The oyster is-estimated to pro? duce over a million eggs, but.the ma}ority are not so prolific. Molluscs art herbivorous, carnivorous.. and omnivorous. The majority of the land specie* feed upon vegetables. Among the marine sppcics, nearly all those whos.e shells arc round-mouthed and have' rib notch or channel .. nt. the inner low«r corner are vegetarian • and. feed upoii growths on, the .rocks. A notable example is the limpet. Where the notch exists, as.wiih the whelks, It may safely be assumed that the animal.is.carnivorous. These feed upon carrion and other shellfish. The classification of mollusca is still a matter of difference of opinion, but the four main classes are, the stomach-footed, the diggingfooted, the axe-footed,: and the beadfooted. •' ; ' • : • ' .-. ■'■

Although niany ..beautiful marine shells are to be found'in New.-Zea-land, they can hardly be compared tp those of tropical countries. ProbabljT one of the most beautiful in form, pearly white in colour, is that of the nautilus, one of the most interesting of the octopods. The female alone possesses a shell, which serves as a nest for the eggs and a nursery, for ,the young. -The male nautilus is only about an inch in length, but the female may occupy a spiral, chambered shell upwards of nine inches in diameter. Tfit animal retains possession of the shell by clasping the sides of the aperture with two dorsal arms.: The shell itself is sculptured with numerous ribs which become nodular as the keel is approach? cd. The beautiful sheila at" one timt wore worth up to £13, but at; the present day little commercial value ir aY tached to them. Probably the best known of New Zealand marine molluscs is the pawa; the internal, colouring .-of metallic tine and green, highly irrideseent with red, purple, and yellow,-al-ways evokes admiration. • The sea-ear, as this species is also known, is found just below low-water mark on rocks, usually in sheltered positions, and BiajT attain a length of six or seven inches. Three species of sea-ears are found'iu New Zealand, one, about half the sire of the pawa, is distinguishable by the well-marked corrugations on the shell, which add much to its beauty, and mark it as probably more beautiful than the pawa.. The third species is named from the more or less distinct :v-shaped streaks of green on the dark brown exterior, but is rather thin, narrower, and longer in campaiison to the previous species. '. ..

Tho limpet is a well-known mollnsi with a conical or tent-shaped shell, and is to be noted tenaciously clinging to the rock by means of its foot, which acts like a sucker. It moves about and grazes on minute vegetation whim the tide is up, always returning 'to'the; same spot before the tide ebbs. Like other members of its family, it grazes by. means of the lingual ribbon, which i» provided with some 2000 hook-lik* teeth. Limpets are used for .bait, and in some places for food. Many eject ies are found in New Zealand water*. The New Zaland olive is a beautiful, smooth, barrel-shaped univalve, with-a short spine coming abruptly to a sharp point; contracted somewhat at the base. The spire is dark brown, with paler spiral stripes, the body whirl ehoeolajt* colour, blurred with blue, .bounded with a narrow white band above and-below with a double white line. It may I*B found during the hot weather in largt numbers on sandy beaches buried in th« sand. At low tides one may iometimeii see hundreds of {hese. mollusea crawling about on the sandy bottom in a few inches of water. It is impossible in:.'aj» article of this description to survey the many species of mollusca to.be found in New Zealand; anyone wishing to gain further knowledge v of these intei* esting animals can do.so by lystematto cally visiting the beachei and rook* of the vicinity, • '■..:• ;.■