the formation of vegetable mould-第6节

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！


we　know　from　Fredericq　that　a　secretion　of　this　kind　is　found　in　
the　intestines　of　worms。　　As　the　leaves　which　are　dragged　into　the　
burrows　are　often　dry　and　shrivelled；　it　is　indispensable　for　their　
disintegration　by　the　unarmed　mouths　of　worms　that　they　should　
first　be　moistened　and　softened；　and　fresh　leaves；　however　soft　and　
tender　they　may　be；　are　similarly　treated；　probably　from　habit。　　
The　result　is　that　they　are　partially　digested　before　they　are　
taken　into　the　alimentary　canal。　　I　am　not　aware　of　any　other　case　
of　extra…stomachal　digestion　having　been　recorded。　　The　boa…
constrictor　is　said　to　bathe　its　prey　with　saliva；　but　this　is　
doubtful；　and　it　is　done　solely　for　the　sake　of　lubricating　its　
prey。　　Perhaps　the　nearest　analogy　may　be　found　in　such　plants　as　
Drosera　and　Dionaea；　for　here　animal　matter　is　digested　and　
converted　into　peptone　not　within　a　stomach；　but　on　the　surfaces　of　
the　leaves。

Calciferous　Glands。These　glands　（see　Fig。　1）；　judging　from　their　
size　and　from　their　rich　supply　of　blood…vessels；　must　be　of　much　
importance　to　the　animal。　　But　almost　as　many　theories　have　been　
advanced　on　their　use　as　there　have　been　observers。　　They　consist　
of　three　pairs；　which　in　the　common　earth…worm　debouch　into　the　
alimentary　canal　in　advance　of　the　gizzard；　but　posteriorly　to　it　
in　Urochaeta　and　some　other　genera。　｛23｝　　The　two　posterior　pairs　
are　formed　by　lamellae；　which；　according　to　Claparede；　are　
diverticula　from　the　oesophagus。　｛24｝　　These　lamellae　are　coated　
with　a　pulpy　cellular　layer；　with　the　outer　cells　lying　free　in　
infinite　numbers。　　If　one　of　these　glands　is　punctured　and　
squeezed；　a　quantity　of　white　pulpy　matter　exudes；　consisting　of　
these　free　cells。　　They　are　minute；　and　vary　in　diameter　from　2　to　
6　microns。　　They　contain　in　their　centres　a　little　excessively　fine　
granular　matter；　but　they　look　so　like　oil　globules　that　Claparede　
and　others　at　first　treated　them　with　ether。　　This　produces　no　
effect；　but　they　are　quickly　dissolved　with　effervescence　in　acetic　
acid；　and　when　oxalate　of　ammonia　is　added　to　the　solution　a　white　
precipitate　is　thrown　down。　　We　may　therefore　conclude　that　they　
contain　carbonate　of　lime。　　If　the　cells　are　immersed　in　a　very　
little　acid；　they　become　more　transparent；　look　like　ghosts；　and　
are　soon　lost　to　view；　but　if　much　acid　is　added；　they　disappear　
instantly。　　After　a　very　large　number　have　been　dissolved；　a　
flocculent　residue　is　left；　which　apparently　consists　of　the　
delicate　ruptured　cell…walls。　　In　the　two　posterior　pairs　of　glands　
the　carbonate　of　lime　contained　in　the　cells　occasionally　
aggregates　into　small　rhombic　crystals　or　into　concretions；　which　
lie　between　the　lamellae；　but　I　have　seen　only　one　case；　and　
Claparede　only　a　very　few　such　cases。

The　two　anterior　glands　differ　a　little　in　shape　from　the　four　
posterior　ones；　by　being　more　oval。　　They　differ　also　conspicuously　
in　generally　containing　several　small；　or　two　or　three　larger；　or　a　
single　very　large　concretion　of　carbonate　of　lime；　as　much　as　1。5　
mm。　in　diameter。　　When　a　gland　includes　only　a　few　very　small　
concretions；　or；　as　sometimes　happens；　none　at　all；　it　is　easily　
overlooked。　　The　large　concretions　are　round　or　oval；　and　
exteriorly　almost　smooth。　　One　was　found　which　filled　up　not　only　
the　whole　gland；　as　is　often　the　case；　but　its　neck；　so　that　it　
resembled　an　olive…oil　flask　in　shape。　　These　concretions　when　
broken　are　seen　to　be　more　or　less　crystalline　in　structure。　　How　
they　escape　from　the　gland　is　a　marvel；　but　that　they　do　escape　is　
certain；　for　they　are　often　found　in　the　gizzard；　intestines；　and　
in　the　castings　of　worms；　both　with　those　kept　in　confinement　and　
those　in　a　state　of　nature。

Claparede　says　very　little　about　the　structure　of　the　two　anterior　
glands；　and　he　supposes　that　the　calcareous　matter　of　which　the　
concretions　are　formed　is　derived　from　the　four　posterior　glands。　　
But　if　an　anterior　gland　which　contains　only　small　concretions　is　
placed　in　acetic　acid　and　afterwards　dissected；　or　if　sections　are　
made　of　such　a　gland　without　being　treated　with　acid；　lamellae　like　
those　in　the　posterior　glands　and　coated　with　cellular　matter　could　
be　plainly　seen；　together　with　a　multitude　of　free　calciferous　
cells　readily　soluble　in　acetic　acid。　　When　a　gland　is　completely　
filled　with　a　single　large　concretion；　there　are　no　free　cells；　as　
these　have　been　all　consumed　in　forming　the　concretion。　　But　if　
such　a　concretion；　or　one　of　only　moderately　large　size；　is　
dissolved　in　acid；　much　membranous　matter　is　left；　which　appears　to　
consist　of　the　remains　of　the　formerly　active　lamellae。　　After　the　
formation　and　expulsion　of　a　large　concretion；　new　lamellae　must　be　
developed　in　some　manner。　　In　one　section　made　by　my　son；　the　
process　had　apparently　commenced；　although　the　gland　contained　two　
rather　large　concretions；　for　near　the　walls　several　cylindrical　
and　oval　pipes　were　intersected；　which　were　lined　with　cellular　
matter　and　were　quite　filled　with　free　calciferous　cells。　　A　great　
enlargement　in　one　direction　of　several　oval　pipes　would　give　rise　
to　the　lamellae。

Besides　the　free　calciferous　cells　in　which　no　nucleus　was　visible；　
other　and　rather　larger　free　cells　were　seen　on　three　occasions；　
and　these　contained　a　distinct　nucleus　and　nucleolus。　　They　were　
only　so　far　acted　on　by　acetic　acid　that　the　nucleus　was　thus　
rendered　more　distinct。　　A　very　small　concretion　was　removed　from　
between　two　of　the　lamellae　within　an　anterior　gland。　　It　was　
imbedded　in　pulpy　cellular　matter；　with　many　free　calciferous　
cells；　together　with　a　multitude　of　the　larger；　free；　nucleated　
cells；　and　these　latter　cells　were　not　acted　on　by　acetic　acid；　
while　the　former　were　dissolved。　　From　this　and　other　such　cases　I　
am　led　to　suspect　that　the　calciferous　cells　are　developed　from　the　
larger　nucleated　ones；　but　how　this　was　effected　was　not　
ascertained。

When　an　anterior　gland　contains　several　minute　concretions；　some　of　
these　are　generally　angular　or　crystalline　in　outline；　while　the　
greater　number　are　rounded　with　an　irregular　mulberry…like　surface。　　
Calciferous　cells　adhered　to　many　parts　of　these　mulberry…like　
masses；　and　their　gradual　disappearance　could　be　traced　while　they　
still　remained　attached。　　It　was　thus　evident　that　the　concretions　
are　formed　from　the　lime　contained　within　the　free　calciferous　
cells。　　As　the　smaller　concretions　increase　in　size；　they　come　into　
contact　and　unite；　thus　enclosing　the　now　functionless　lamellae；　
and　by　such　steps　the　formation　of　the　largest　concretions　could　be　
followed。　　Why　the　process　regularly　takes　place　in　the　two　
anterior　glands；　and　only　rarely　in　the　four　posterior　glands；　is　
quite　unknown。　　Morren　says　that　these　glands　disappear　during　the　
winter；　and　I　have　seen　some　instances　of　this　fact；　and　others　in　
which　either　the　anterior　or　posterior　glands　were　at　this　season　
so　shrunk　and　empty；　that　they　could　be　distinguished　only　with　
much　difficulty。

With　respect　to　the　function　of　the　calciferous　glands；　it　is　
probable　that　they　primarily　serve　as　organs　of　excretion；　and　
secondarily　as　an　aid　to　digestion。　　Worms　consume　many　fallen　
leaves；　and　it　is　known　that　lime　goes　on　accumulating　in　leaves　
until　they　drop　off　the　parent…plant；　instead　of　being　re…absorbed　
into　the　stem　or　roots；　like　various　other　organic　and　inorganic　
substances。　｛25｝　　The　ashes　of　a　leaf　of　an　acacia　have　been　known　
to　contain　as　much　as　72　per　cent。　of　lime。　　Worms　therefore　would　
be　liable　to　become　charged　with　this　earth；　unless　there　were　some　
special　means　for　its　excretion；　and　the　calciferous　glands　are　
well　adapted　for　this　purpose。　　The　worms　which　live　in　mould　close　
over　the　chalk；　often　have　their　intestines　filled　with　this　
substance；　and　their　castings　are　almost　white。　　Here　it　is　evident　
that　the　supply　of　calcareous　matter　must　be　super…abundant。　　
Nevertheless　with　several　worms　collected　on　such　a　site；　the　
calciferous　glands　contained　as　many　free　calciferous　cells；　and　
fully　as　many　and　large　concretions；　as　did　the　glands　of　worms　
which　lived　where　there　was　little　or　no　lime；　and　this　indicates　
that　the　lime　is　an　excretion；　and　not　a　secretion　poured　into　the　
alimentary　canal　for　some　special　purpose。

On　the　other　hand；　the　following　considerations　render　it　highly　
probable　that　the　carbonate　of　lime；　which　is　excreted　by　the　
glands；　aids　the　digestive　process　under　ordinary　circumstances。　　
Leaves　during　their　decay　generate　an　abundance　of　various　kinds　of　
acids；　which　have　been　grouped　together　under　the　term　of　humus　
acids。　　We　shall　have　to　recur　to　this　subject　in　our　fifth　
chapter；　and　I　need　here　only　say　that　these　acids　act　strongly　on　
carbonate　of　lime。　　The　half…decayed　leaves　which　are　swallowed　in　
such　large　quantities　by　worms　would；　therefore；　after　they　have　
been　moistened　and　triturated　in　the　alimentary　canal；　be　apt　to　
produce　such　acids。　　And　in　the　case　of　several　worms；　the　contents　
of　the　alimentary　canal　were　found　to　be　plainly　acid；　as　shown　by　
litmus　paper。