#sac  a structure resembling a bag in an animal
  supertype:  bodily_cavity__cavity__cavum  (anatomy) a natural hollow or sinus within the body
  subtype:  yolk_sac  membranous structure enclosing the yolk of eggs in birds reptiles marsupials and some fishes; circulates nutrients to the developing embryo
  subtype:  vitelline_sac__vitellinesac__yolk_sac__umbilical_vesicle__umbilicalvesicle__vesicula_umbilicus  membranous structure that functions as the circulatory system in mammal embryos until the heart becomes functional
  subtype:  amnion__amniotic_sac__amnios  thin innermost membranous sac enclosing the developing embryo in higher vertebrates (Amniota: reptiles; birds; mammals)
  subtype:  chorion  outermost membranous sac enclosing the embryo in higher vertebrates (reptiles; birds; mammals)
  subtype:  coelenteron  the saclike body cavity of a coelenterate
  subtype:  air_bladder__swim_bladder  an air-filled sac near the spinal column in many fishes that helps maintain buoyancy
  subtype:  air_sac  any of the membranous air-filled extensions of the lungs of birds
  subtype:  air_sac.sac  any of the thin-walled extensions of the tracheae of insects
  subtype:  saccule__sacculus  a small sc or pouch (especially the smaller chamber of the membranous labyrinth)
  subtype:  vesica__bladder  a distensible membranous sac (usually containing liquid or gas)
     subtype:  urinary_bladder__urinarybladder  a membranous sac for temporary retention of urine
  subtype:  acinus  one of the small sacs or saclike dilations in a compound gland
  subtype:  bursa  a small fluid-filled sac located between movable parts of the body especially at joints
  subtype:  cisterna__cistern  a sac or cavity containing fluid especially lymph or cerebrospinal fluid
  subtype:  pouch.sac__pocket  (anatomy) saclike structure in any of various animals (as a marsupial or gopher or pelican)
     subtype:  gizzard__ventriculus__ventriculu__gastric_mill__gastricmill  thick-walled muscular pouch below the crop in many birds and reptiles for grinding food
     subtype:  utricle__utriculus__utriculu  a small pouch into which the semicircular canals open
     subtype:  atrial_auricle__auricle__auriculaatrii  a small conical pouch projecting from the upper anterior part of each atrium of the heart
     subtype:  auricula__auricular_appendage__auricularappendage__auricular_appendix__auricularappendix  a pouch projecting from the top front of each atrium of the heart
     subtype:  cheek_pouch__cheekpouch  a membranous pouch inside the mouth of many rodents (as a gopher)
     subtype:  marsupium  an external abdominal pouch in most marsupials where newborn offspring are suckled
     subtype:  scrotum  the external pouch that contains the testes
  subtype:  vesicle__cyst  a small anatomically normal sac or bladderlike structure (especially one containing fluid)
     subtype:  lacrimal_sac__tear_sac__dacryocyst  either of the two dilated ends of the lacrimal ducts at the nasal ends of the eyes that fill with tears secreted by the lacrimal glands
     subtype:  Golgi_body__Golgi_apparatus__Golgi_complex__dictyosome  a net-like structure in the cytoplasm of animal cells (especially in those cells that produce secretions)
     subtype:  blister__bulla__bleb  (pathology) an elevation of the skin filled with serous fluid
        subtype:  water_blister__waterblister  blister containing a nonpurulent clear watery content
        subtype:  blood_blister__bloodblister  blister containing blood or bloody serum usually caused by an injury
     subtype:  follicle  any small spherical group of cells containing a cavity
        subtype:  hair_follicle  a small tubular cavity containing the root of a hair; small muscles and sebaceous glands are associated with hair follicles
        subtype:  Graafian_follicle  a vascular body in a mammalian ovary enclosing a developing egg
     subtype:  seminal_vesicle__seminalvesicle  adds nutrient fluid to semen during ejaculation
  subtype:  alveolus__air_sac__air_cell__aircell  a tiny sac for holding air in the lungs; formed by the terminal dilation of tiny air passageways
  subtype:  pericardial_sac__pericardialsac  the membrane surrounding the heart

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