Text-book of comparative anatomy . upperpart of the body. The lefthalf of the section is omitted. The mesoderm of the sponges is representedby a middle layer of connective tissue, chieflygelatinous, with cells imbedded in it. Thelatter are either spindle-shaped or star-shaped, occasionally of them often contain colouring matter (pigment cells); otherscan move like Amcebce (migratory cells). Long spindle-shaped andfinely-branched cells occasionally lie concentrically at the commence-ment of the afferent canals, and no doubt serve as contractile cellsfor closing the pores. The me
Text-book of comparative anatomy . upperpart of the body. The lefthalf of the section is omitted. The mesoderm of the sponges is representedby a middle layer of connective tissue, chieflygelatinous, with cells imbedded in it. Thelatter are either spindle-shaped or star-shaped, occasionally of them often contain colouring matter (pigment cells); otherscan move like Amcebce (migratory cells). Long spindle-shaped andfinely-branched cells occasionally lie concentrically at the commence-ment of the afferent canals, and no doubt serve as contractile cellsfor closing the pores. The mesodermal connective tissue is, in sponges, the place of for-mation of the very varied skeletal structures. These consist eitherof carbonate of lime, or of silica, or of horn known as spongin. G4 COMPARATIVE ANATOMY CHAF. Skeletons of silica and of horn fibres are found combined. The sili-ceous or calcareous skeletons consist of small bodies of extraordinarilydifferent shapes, the so-called spicules, most probably formed in the ec. FIG. 59.—Part of a section through Halisarca lobularis, after F. E. Schulze. ec, Ectodermalpavement epithelium ; gh, gastral cavity ; in, mesoderm ; p, pores ; gk, ciliated chambers ; zk, affer-ent canals ; o, eggs in different stages of segmentation. cells. There are uniradiate, triradiate, quadriradiate, sexiradiate, multi-radiate forms, stars, spheres, etc. The skeleton of a sponge may con-sist of only one sort ofspicule, or two or moresorts may occur together. The single spicules lieeither loosely near eachother, or are cementedtogether into coherentframeworks. The same isthe case with horn ordinary bath spongeis only a framework of suchhorn fibres; it is merelythe skeleton of a marineanimal (Fig. 56).A nervous system is not yet with certainty proved to exist in thePorifcm. Reproduction is either asexual or sexual. Asexual reproduction takes place by external or internal buddingor gemmation. External gemmation.—A sponge may put out
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectanatomycomparative
