the formation of vegetable mould-第2章

按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！


McIntosh；　however；　found　worm…castings　at　a　height　of　1500　feet　on　
Schiehallion　in　Scotland。　　They　are　numerous　on　some　hills　near　
Turin　at　from　2000　to　3000　feet　above　the　sea；　and　at　a　great　
altitude　on　the　Nilgiri　Mountains　in　South　India　and　on　the　
Himalaya。

Earth…worms　must　be　considered　as　terrestrial　animals；　though　they　
are　still　in　one　sense　semi…aquatic；　like　the　other　members　of　the　
great　class　of　annelids　to　which　they　belong。　　M。　Perrier　found　
that　their　exposure　to　the　dry　air　of　a　room　for　only　a　single　
night　was　fatal　to　them。　　On　the　other　hand　he　kept　several　large　
worms　alive　for　nearly　four　months；　completely　submerged　in　water。　
｛10｝　　During　the　summer　when　the　ground　is　dry；　they　penetrate　to　a　
considerable　depth　and　cease　to　work；　as　they　do　during　the　winter　
when　the　ground　is　frozen。　　Worms　are　nocturnal　in　their　habits；　
and　at　night　may　be　seen　crawling　about　in　large　numbers；　but　
usually　with　their　tails　still　inserted　in　their　burrows。　　By　the　
expansion　of　this　part　of　their　bodies；　and　with　the　help　of　the　
short；　slightly　reflexed　bristles；　with　which　their　bodies　are　
armed；　they　hold　so　fast　that　they　can　seldom　be　dragged　out　of　the　
ground　without　being　torn　into　pieces。　｛11｝　　During　the　day　they　
remain　in　their　burrows；　except　at　the　pairing　season；　when　those　
which　inhabit　adjoining　burrows　expose　the　greater　part　of　their　
bodies　for　an　hour　or　two　in　the　early　morning。　　Sick　individuals；　
which　are　generally　affected　by　the　parasitic　larvae　of　a　fly；　must　
also　be　excepted；　as　they　wander　about　during　the　day　and　die　on　
the　surface。　　After　heavy　rain　succeeding　dry　weather；　an　
astonishing　number　of　dead　worms　may　sometimes　be　seen　lying　on　the　
ground。　　Mr。　Galton　informs　me　that　on　one　such　occasion　（March；　
1881）；　the　dead　worms　averaged　one　for　every　two　and　a　half　paces　
in　length　on　a　walk　in　Hyde　Park；　four　paces　in　width。　　He　counted　
no　less　than　45　dead　worms　in　one　place　in　a　length　of　sixteen　
paces。　　From　the　facts　above　given；　it　is　not　probable　that　these　
worms　could　have　been　drowned；　and　if　they　had　been　drowned　they　
would　have　perished　in　their　burrows。　　I　believe　that　they　were　
already　sick；　and　that　their　deaths　were　merely　hastened　by　the　
ground　being　flooded。

It　has　often　been　said　that　under　ordinary　circumstances　healthy　
worms　never；　or　very　rarely；　completely　leave　their　burrows　at　
night；　but　this　is　an　error；　as　White　of　Selborne　long　ago　knew。　　
In　the　morning；　after　there　has　been　heavy　rain；　the　film　of　mud　or　
of　very　fine　sand　over　gravel…walks　is　often　plainly　marked　with　
their　tracks。　　I　have　noticed　this　from　August　to　May；　both　months　
included；　and　it　probably　occurs　during　the　two　remaining　months　of　
the　year　when　they　are　wet。　　On　these　occasions；　very　few　dead　
worms　could　anywhere　be　seen。　　On　January　31；　1881；　after　a　long…
continued　and　unusually　severe　frost　with　much　snow；　as　soon　as　a　
thaw　set　in；　the　walks　were　marked　with　innumerable　tracks。　　On　one　
occasion；　five　tracks　were　counted　crossing　a　space　of　only　an　inch　
square。　　They　could　sometimes　be　traced　either　to　or　from　the　
mouths　of　the　burrows　in　the　gravel…walks；　for　distances　between　2　
or　3　up　to　15　yards。　　I　have　never　seen　two　tracks　leading　to　the　
same　burrow；　nor　is　it　likely；　from　what　we　shall　presently　see　of　
their　sense…organs；　that　a　worm　could　find　its　way　back　to　its　
burrow　after　having　once　left　it。　　They　apparently　leave　their　
burrows　on　a　voyage　of　discovery；　and　thus　they　find　new　sites　to　
inhabit。

Morren　states　｛12｝　that　worms　often　lie　for　hours　almost　motionless　
close　beneath　the　mouths　of　their　burrows。　　I　have　occasionally　
noticed　the　same　fact　with　worms　kept　in　pots　in　the　house；　so　that　
by　looking　down　into　their　burrows；　their　heads　could　just　be　seen。　　
If　the　ejected　earth　or　rubbish　over　the　burrows　be　suddenly　
removed；　the　end　of　the　worm's　body　may　very　often　be　seen　rapidly　
retreating。　　This　habit　of　lying　near　the　surface　leads　to　their　
destruction　to　an　immense　extent。　　Every　morning　during　certain　
seasons　of　the　year；　the　thrushes　and　blackbirds　on　all　the　lawns　
throughout　the　country　draw　out　of　their　holes　an　astonishing　
number　of　worms；　and　this　they　could　not　do；　unless　they　lay　close　
to　the　surface。　　It　is　not　probable　that　worms　behave　in　this　
manner　for　the　sake　of　breathing　fresh　air；　for　we　have　seen　that　
they　can　live　for　a　long　time　under　water。　　I　believe　that　they　lie　
near　the　surface　for　the　sake　of　warmth；　especially　in　the　morning；　
and　we　shall　hereafter　find　that　they　often　coat　the　mouths　of　
their　burrows　with　leaves；　apparently　to　prevent　their　bodies　from　
coming　into　close　contact　with　the　cold　damp　earth。　　It　is　said　
that　they　completely　close　their　burrows　during　the　winter。

Structure。A　few　remarks　must　be　made　on　this　subject。　　The　body　
of　a　large　worm　consists　of　from　100　to　200　almost　cylindrical　
rings　or　segments；　each　furnished　with　minute　bristles。　　The　
muscular　system　is　well　developed。　　Worms　can　crawl　backwards　as　
well　as　forwards；　and　by　the　aid　of　their　affixed　tails　can　retreat　
with　extraordinary　rapidity　into　their　burrows。　　The　mouth　is　
situated　at　the　anterior　end　of　the　body；　and　is　provided　with　a　
little　projection　（lobe　or　lip；　as　it　has　been　variously　called）　
which　is　used　for　prehension。　　Internally；　behind　the　mouth；　there　
is　a　strong　pharynx；　shown　in　the　accompanying　diagram　（Fig。　1）　
which　is　pushed　forwards　when　the　animal　eats；　and　this　part　
corresponds；　according　to　Perrier；　with　the　protrudable　trunk　or　
proboscis　of　other　annelids。　　The　pharynx　leads　into　the　
oesophagus；　on　each　side　of　which　in　the　lower　part　there　are　three　
pairs　of　large　glands；　which　secrete　a　surprising　amount　of　
carbonate　of　lime。　　These　calciferous　glands　are　highly　remarkable；　
for　nothing　like　them　is　known　in　any　other　animal。　　Their　use　will　
be　discussed　when　we　treat　of　the　digestive　process。　　In　most　of　
the　species；　the　oesophagus　is　enlarged　into　a　crop　in　front　of　the　
gizzard。　　This　latter　organ　is　lined　with　a　smooth　thick　chitinous　
membrane；　and　is　surrounded　by　weak　longitudinal；　but　powerful　
transverse　muscles。　　Perrier　saw　these　muscles　in　energetic　action；　
and；　as　he　remarks；　the　trituration　of　the　food　must　be　chiefly　
effected　by　this　organ；　for　worms　possess　no　jaws　or　teeth　of　any　
kind。　　Grains　of　sand　and　small　stones；　from　the　1/20　to　a　little　
more　than　the　1/10　inch　in　diameter；　may　generally　be　found　in　
their　gizzards　and　intestines。　　As　it　is　certain　that　worms　swallow　
many　little　stones；　independently　of　those　swallowed　while　
excavating　their　burrows；　it　is　probable　that　they　serve；　like　
mill…stones；　to　triturate　their　food。　　The　gizzard　opens　into　the　
intestine；　which　runs　in　a　straight　course　to　the　vent　at　the　
posterior　end　of　the　body。　　The　intestine　presents　a　remarkable　
structure；　the　typhlosolis；　or；　as　the　old　anatomists　called　it；　an　
intestine　within　an　intestine；　and　Claparede　｛13｝　has　shown　that　
this　consists　of　a　deep　longitudinal　involution　of　the　walls　of　the　
intestine；　by　which　means　an　extensive　absorbent　surface　is　gained。

The　circulatory　system　is　well　developed。　　Worms　breathe　by　their　
skin；　as　they　do　not　possess　any　special　respiratory　organs。　　The　
two　sexes　are　united　in　the　same　individual；　but　two　individuals　
pair　together。　　The　nervous　system　is　fairly　well　developed；　and　
the　two　almost　confluent　cerebral　ganglia　are　situated　very　near　to　
the　anterior　end　of　the　body。

Senses。Worms　are　destitute　of　eyes；　and　at　first　I　thought　that　
they　were　quite　insensible　to　light；　for　those　kept　in　confinement　
were　repeatedly　observed　by　the　aid　of　a　candle；　and　others　out　of　
doors　by　the　aid　of　a　lantern；　yet　they　were　rarely　alarmed；　
although　extremely　timid　animals。　　Other　persons　have　found　no　
difficulty　in　observing　worms　at　night　by　the　same　means。　｛14｝

Hoffmeister；　however；　states　｛15｝　that　worms；　with　the　exception　of　
a　few　individuals；　are　extremely　sensitive　to　light；　but　he　admits　
that　in　most　cases　a　certain　time　is　requisite　for　its　action。　　
These　statements　led　me　to　watch　on　many　successive　nights　worms　
kept　in　pots；　which　were　protected　from　currents　of　air　by　means　of　
glass　plates。　　The　pots　were　approached　very　gently；　in　order　that　
no　vibration　of　the　floor　should　be　caused。　　When　under　these　
circumstances　worms　were　illuminated　by　a　bull's…eye　lantern　having　
slides　of　dark　red　and　blue　glass；　which　intercepted　so　much　light　
that　they　could　be　seen　only　with　some　difficulty；　they　were　not　at　
all　affected　by　this　amount　of　light；　however　long　they　were　
exposed　to　it。　　The　light；　as　far　as　I　could　judge；　was　brighter　
than　that　from　the　full　moon。　　Its　colour　apparently　made　no　
difference　in　the　result。　　When　they　were　illuminated　by　a　candle；　
or　even　by　a　bright　paraffin　lamp；　they　were　not　usually　affected　
at　first。　　Nor　were　they　when　the　light　was　alternately　admitted　
and　shut　off。　　Sometimes；　however；　they　behaved　very　differently；　
for　as　soon　as　the　light　fell　on　them；　they　withdrew　into　their　
burrows　with　almost　instantaneous　rapidity。　　This　occurred　perhaps　
once　out　of　a　dozen　times。　　When　they　did　not　withdraw　instantly；　
they　often　raised　the　anterior　tapering　ends　of　their　bodies　from　
the　ground；　as　if　their　attention　was　aroused　or　as　if　surprise　was　
felt；　or　they　moved　their　bodies　from　side　to　side　as　if　feeling　
for　some　object。　　They　appeared　distressed　by　the　light；　but　I　
doubt　whether　this　was　really　the　case；　for　on　two　occasions　after　
withdrawing　slowly；　they　remained　for　a　long　time　with　their　
anterior　extremities　protruding　a　little　from　the　mouths　of　their　
burrows；　in　which　position　they　were　ready　for　instant　and　complete　
withdrawal。

When　the　light　from　a　candle　was　concentrated　by　means　of　a　large　
lens　on　the　anterior　extremity；　they　generally　withdrew　instantly；　
but　this　concentrated　light　failed　to　act　perhaps　once　out　of　half　
a　dozen　trials。　　The　light　was　on　one　occasion　concentrated　on　a　
worm　lying　beneath　water　in　a　saucer；　and　it　instantly　withdrew　
into　its　burrow。　　In　all　cases　the　duration　of　the　light；　unless　
extremely　feeble；　made　a　great　difference　in　the　result；　for　worms　
left　exposed　before　a　paraffin　lamp　or　a　candle　invariably　
retreated　into　their　burrows　within　from　five　to　fifteen　minutes；　
and　if　in　the　evening　the　pots　were　illuminated　before　the　worms　
had　come　out　of　their　burrows；　they　failed　to　appear。

From　the　foregoing　facts　it　is　evident　that　light　affects　worms　by　
its　intensity　and　by　its　duration。　　It　is　only　the　anterior　
extremity　of　the　body；　where　the　cerebral　ganglia　lie；　which　is　
affected　by　light；　as　Hoffmeister　asserts；　and　as　I　observed　on　
many　occasions。　　If　this　part　is　shaded；　other　parts　of　the　body　
may　be　fully　illuminated；　and　no　effect　will　be　produced。　　As　these　
animals　have　no　eyes；　we　must　suppose　that　the　light　passes　through　
their　skins；　and　in　some　manner　excites　their　cerebral　ganglia。　　It　
appeared　at　first　probable　that　the　different　manner　in　which　they　
were　affected　on　different　occasions　might　be　expla