Outlines of zoology . theupper pole. The young sponge is nowFig. 62.—Diagrammatic re- in an Ascon stage, from which, bypresentation of development the outgrowth of the inner layer intoof Oscarella lobularis.— radial chambers, it passes into theAfter Heider. permanent Sycon form, grows into a Bl., Free-swimming blastula .with cylinder, and becomes differentiated inflagella; G., gastrula settled detail (Fig. 61). (b) In Oscarella (Halisarca) lobularis(Fig. 62), a sponge without any skeleton,the ovum segments equally into ablastula, which is flagellate all free-swimming stage may be in-
Outlines of zoology . theupper pole. The young sponge is nowFig. 62.—Diagrammatic re- in an Ascon stage, from which, bypresentation of development the outgrowth of the inner layer intoof Oscarella lobularis.— radial chambers, it passes into theAfter Heider. permanent Sycon form, grows into a Bl., Free-swimming blastula .with cylinder, and becomes differentiated inflagella; G., gastrula settled detail (Fig. 61). (b) In Oscarella (Halisarca) lobularis(Fig. 62), a sponge without any skeleton,the ovum segments equally into ablastula, which is flagellate all free-swimming stage may be in-vaginated from either pole to form ahemispherical gastrula, which settles mouth downwards. Pores, anosculum, and the mesogloea are formed as before, and the inner layerbecomes folded into flagellate chambers. The main features of sponge embryology are thus summarised byMinchin :— I. The larva is composed of three classes of cell-elements: (i)Columnar flagellated cells, forming the outer covering or localised at. down. Next figure shows folding of innerlayer (^m.) ; Ec.^ outer layer. Lowest figure shows radial cham-bers (); Mesogtea (Mg.);inhalant pore (-P.); exhalantosculum {p.). CLASSIFICATION. 133 the anterior pole; {2) rounded, more or less amoeboid elements, rarelyflagellated, forming the inner mass or aggregated at the posteriorpole; and {3) the archaeocytes, usually scattered in the inner mass,and often represented by undifferentiated blastomeres. . II. The larva fixes and undergoes a metamorphosis whereby theflagellated cells become placed in the interior, while the cells of theinner mass come to surround them completely. III. (l) The flagellated cells of the larva become the choanocytesof the adult (gastral layer), acquiring a collar ; . . (2) the inner massgives rise to the dermal layer in its entirety; ... (3) the archseocytesbecome the wandering cells of the adult, from which the reproductivecells arise. It is interesting to note that the primitive germ-cell
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Keywords: ., bookcentury1900, bookdecade1910, bookidcu3192, booksubjectzoology
