Outlines of zoology . ed larva. Withthe exception of one family,all are marine. Description of a simplesponge. — A very simplesponge, such as Ascetta, isa hollow vase, moored at one end to rock or seaweed, with a large exhalant aperture at the opposite pole, and with numerous minute inhalant pores penetrating the walls. In the calcareous sponges, the pores are minute perforations in single cells (porocytes). The walls consist of—(i) a flat covering layer; (2) a mesoglcea containing triradiate calcareous spicules, phagocytes, and reproduc-tive elements; and (3) a layer lining the central cavity
Outlines of zoology . ed larva. Withthe exception of one family,all are marine. Description of a simplesponge. — A very simplesponge, such as Ascetta, isa hollow vase, moored at one end to rock or seaweed, with a large exhalant aperture at the opposite pole, and with numerous minute inhalant pores penetrating the walls. In the calcareous sponges, the pores are minute perforations in single cells (porocytes). The walls consist of—(i) a flat covering layer; (2) a mesoglcea containing triradiate calcareous spicules, phagocytes, and reproduc-tive elements; and (3) a layer lining the central cavity, and composed of collared flagellate cells, like some of the monad Infusorians (cf. Fig. 55)-More complicated forms.—But a description of a simple sponge like Ascetta conveys little idea of the structure of a complex form such as the bath-sponge {Euspongia). Let us consider the origin of complications. {a) Sponges—-long regarded as plants—are plant-like in being sedentary and passive. They seem also to feed. Flu. 59. — Section of asponge.—After F. Showing inhalant canals,flagellate chambers, agastrula forming in themesogloea, etc. STRUCTURE OF SPONGES. 127 easily and well. Like plants, they form buds, the outcomeof surplus nourishment. These buds, like the suckers ofa rose-bush, often ac-quire some apparentindependence, and- thesponge looks like manyvases, not like , as they growthese buds may fuse,like the branches of atree tied closely to-gether. Thus the struc-ture becomes more in-tricate. ib) In the simplesponge the cavity of thevase is completely linedby the collared flagellatecells {Ascon type). Butthe inner layer may growout into radial chambersto which the choan-ocytes are restricted{Syion type), and thewalls of these may alsobe folded into side aisles(Leucon type). The out-growing ? of the innerlayer into the mesogloeamay be continued evenfurther, and the cellsmay become pavement-like except in theminute flagellate cham-bers, where
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Keywords: ., bookcentury1900, bookdecade1910, bookidcu3192, booksubjectzoology
