第 37 节
作者:精灵王      更新:2021-04-30 17:23      字数:9322
  retreats   within   its   cell   by   an   independent   action;   and   when   protruded
  puts   forth   a  circle   of   tentacles   whose   motion   round   the   mouth   is   the
  means   of     securing   nourishment。        There   are;   however;   peculiarities   in
  the     structure     of   the   Polyzoa     which    seem     to  remove      them    from
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  Zoophytology to a place in the system of nature more nearly                    connected
  with Molluscan   types。        Some   of   them  come so   near to the        compound
  ascidians that they have been termed; as an order;             〃Zoophyta ascidioida。〃
  The simplest form of polype is that of a fleshy bag open at one                  end;
  surmounted by a circle of contractile threads or fingers               called tentacles。
  The plate shows; on a very minute scale; at              figs。 1; 3; and 6; several of
  these    little  polypiform     bodies    protruding     from    their  cells。   But    the
  Hydra or Fresh…water Polype            has no cell; and is quite unconnected with
  any   root   thread;   or   with  other   individuals   of   the   same   species。    It   is
  perfectly   free;   and   so   simple   in   its   structure;   that   when   the   sac   which
  forms     its  body    is  turned    inside   out  it  will  continue    to  perform    the
  functions     of    life   as  before。     The    greater    part;  however;     of   these
  Hydraform         Polypes;      although     equally    simple     as   individuals;    are
  connected      in  a   compound        life  by  means     of  their  variously    formed
  POLYPIDOM; as the            branched system of cells is termed。           The Hydroid
  Zoophytes are       represented in the first plate by the following examples。
  HYDROIDA。
  SERTULARIA ROSEA。               PL。 I。 FIG。 6。
  A species which has the cells in pairs on opposite sides of the               central
  tube; with the openings turned outwards。             In the more      enlarged figure is
  seen   a   septum  across   the   inner   part   of   each   cell which   forms   the   base
  upon which the polype rests。           Fig。 6 B     indicates the natural size of the
  piece of branch represented; but           it must be remembered that this is only
  a small portion of the       bushy shrub。
  CAMPANULARIA SYRINGA。                    PL。 I。 FIG。 8。
  This    Zoophyte      twines     itself  parasitically    upon     a   species    of
  Sertularia。     The cells in this species are thrown out at irregular            intervals
  upon     flexible   stems   which    are   wrinkled    in  rings。    They     consist    of
  lengthened; cylindrical; transparent vases。
  CAMPANULARIA VOLUBILIS。                    PL。 I。 FIG。 9。
  A still more beautiful species; with lengthened foot…stalks ringed                at
  each end。      The polype is remarkable for the protrusion and                contractile
  power of its lips。     It has about twenty knobbed           tentacula。
  POLYZOA。
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  Among   Polyzoa   the   animal's   body   is   coated   with   a   membraneous
  covering;      like   that   of   the   Tunicated      Mollusca;      but   which     is   a
  continuation of the edge of the cell; which doubles back upon the                    body
  in such a manner that when the animal protrudes from its cell                   it pushes
  out the flexible membrane just as one would turn inside                out the finger of
  a   glove。   This   oneness of   cell   and   polype   is  a  distinctive   character  of
  the   group。    Another      is  the  higher   organization      of  the  internal   parts。
  The mouth; surrounded by           tentacles; leads by gullet and gizzard through
  a   channel   into   a  digesting   stomach;   from   which   the   rejectable   matter
  passes upwards        through an intestinal canal till it is discharged near the
  mouth。       The     tentacles   also  differ  much     from   those   of  true  Polypes。
  Instead     of being fleshy and contractile; they are rather stiff; resembling
  spun glass; set on the sides with vibrating cilia; which by their              motion up
  one side and down the other of each tentacle; produce a                   current which
  impels     their  living   food   into  the   mouth。     When     these    tentacles    are
  withdrawn;      they   are   gathered    up   in  a  bundle;    like  the   stays    of  an
  umbrella。      Our Plate I。 contains the following examples             of Polyzoa。
  VALKERIA CUSCUTA。 PL。 I。 FIG。 3。
  From a group in one of Mr。 Lloyd's vases。               Fig。 3 A is the natural
  size of the central group of cells; in a specimen coiled round a                  thread…
  like   weed。     Underneath       this  is  the   same    portion   enlarged。       When
  magnified to this apparent size; the cells could be seen in             different states;
  some closed; and others with their bodies              protruded。     When magnified
  to 3 D; we could pleasantly watch the            gradual eversion of the membrane;
  then the points   of the  tentacles       slowly appearing; and   then; when  fully
  protruded; suddenly        expanding into a bell…shaped circle。           This was their
  usual    appearance; but sometimes they could be noticed bending inwards;
  as    in fig。 3 C; as if to imprison some living atom of importance。                  Fig。
  B    represents   two    tentacles;  showing     the   direction   in  which    the   cilia
  vibrate。
  CRISIA DENTICULATA。               PL。 I。 FIG。 4。
  I have only drawn the cells from a prepared specimen。                The polypes
  are like those described above。
  GEMELLARIA LORICATA。                  PL。 I。 FIG。 5。
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  Here   the   cells   are   placed   in   pairs;   back   to   back。 5   A   is   a   very
  small portion on the natural scale。
  CELLULARIA CILIATA。                Pl。 I。 FIG。 7
  The   cells   are   alternate   on   the   stem;   and   are   curiously   armed   with
  long   whip…like   cilia   or   spines。   On   the   back   of   some   of   the   cells is   a
  very     strange    appendage;       the   use    of   which     is   not   with    certainty
  ascertained。      It   is   a   minute   body;   slightly   resembling   a   vulture's head;
  with    a   movable     lower    beak。     The    whole     head   keeps    up   a   nodding
  motion;      and   the   movable     beak    occasionally      opens    widely;    and   then
  suddenly snaps to with a jerk。            It has been seen to hold an           animalcule
  between      its  jaws    till  the  latter   has   died;   but   it  has   no    power     to
  communicate the prey to the polype in its cell or to               swallow and digest it
  on its own account。          It is certainly not an       independent parasite; as has
  been supposed; and yet its purpose in              the animal economy is a mystery。
  Mr。   Gosse   conjectures   that   its    use   may  be;   by   holding   animalcules   till
  they    die   and   decay;   to   attract    by  their   putrescence     crowds     of  other
  animalcules;       which    may     thus    be   drawn    within    the  influence     of  the
  polype's   ciliated     tentacles。     Fig。   7   B   shows   the   form   of   one   of   these
  〃birds' heads;〃      and fig。 7 C; its position on the cell。
  FLUSTRA LINEATA。               PL。 I。 FIG。 1。
  In   Flustrae;    the  cells   are   placed    side   by   side   on   an   expanded
  membrane。        Fig。 1 represents the general appearance of a species                which
  at least resembles F。 lineata as figured in Johnston's work。                    It is spread
  upon a Fucus。        Fig。 A is an enlarged view of the          cells。
  FLUSTRA FOLIACEA。                PL。 I。 FIG。 2。
  We figure a frond or two of the common species; which has cells on
  both    sides。    It  is  rarely   that  the   polypes    can   be   seen   in  a  state   of
  expansion。
  SERIALARIA LENDIGERA。                   PL。 I。 fig。 10。
  NOTAMIA BURSARIA。                  PL。    I。 fig。 11。
  The   〃tobacco…pipe〃〃   appendages;   fig。   11         B;   are   of   unknown    use:
  they are probably analogous to the birds' heads in the Cellularae。
  PLATE V。
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