Elementary text-book of zoology, general part and special part: protozoa to insecta . elementarytextbo00clau Year: 1892 gelatinous Sponges or Myxosjiongia, is composed of horny fibres or silicious or calcareous spicules. The horny fibres form, without exception, anastomosing networks of varying degrees of thickness, and present a lamellated structure (fig. 158), which indicates that they are formed of a number of layers. They are formed by excretion as hardening portions of sarcode. The calcareous needles (fig. 159) are simple or three- and four- rayed spicules, and take their origin, as do t
Elementary text-book of zoology, general part and special part: protozoa to insecta . elementarytextbo00clau Year: 1892 gelatinous Sponges or Myxosjiongia, is composed of horny fibres or silicious or calcareous spicules. The horny fibres form, without exception, anastomosing networks of varying degrees of thickness, and present a lamellated structure (fig. 158), which indicates that they are formed of a number of layers. They are formed by excretion as hardening portions of sarcode. The calcareous needles (fig. 159) are simple or three- and four- rayed spicules, and take their origin, as do the silicious structures, in the interior of cells. The silicious spicules present, however, an extraordinary variety of form: some of them constitute a connected frame- Avork of silicious fibres, and others are free silicious bodies with simple or branched central canals (fig. IGO). The latter are found in the form of needles, spindles, cylinders, hooks, anchors, wheels, and crosses, and arise in nucleated cells, pro- bably as deposits round a hardening of organic matter (central fibre). In order to understand the morphology of the Spongiaria we must begin by examining the structure of a young Sponge, which proceeds from the fixed larva. The young Sponge, after the formation of a cihated gastric cavity and an ex- halent opening or osculum, has the form of a simple hollow tube, the walls of which are pierced by pores for the passage of small food particles suspended in the water (fig. 152). In this stage we can distinguish three layers — (1) an entoderm, formed of elongated flagellated cells; (2) a mesoderm, the skeleto- genous cell layer, the structure of which recalls connective tissue; and (3) an ectoderm, which forms the outer layer of the Sponge, and consists of a flat epithelium. The cylindrical cells of the endo- derm possess at their free ends surrounding the flagellum a delicate Fig. 158.—Piece of network of horny fibres from Euspongia equina. Fig. 159.—Calcare
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