Page:EB1911 - Volume 25.djvu/739

bath sponge appears as a dark-coloured, irregular or sometimes cup-shaped mass attached by the under surface to the sea-bottom. The outer surface is covered by a skin or dermal membrane, elevated in innumerable minute conuli by the growing apices of the primary skeleton fibres. This skin is pierced by a vast number of inhalant dermal pores of microscopic size, and by a much smaller number of comparatively large vents or oscula. When the sponge is removed from the water the soft tissues rapidly decay and leave behind only the elastic “horny” skeleton, which is what we usually speak of under the name “sponge.” It consists of a very close network of spongin fibres (closely resembling silk in chemical composition), some of which, known as primaries, run towards the surface at fairly regular intervals, while others, known as secondary fibres, connect the primaries in all directions and themselves branch and anastomose freely. The primary fibres contain particles of sand or foreign spicules which are taken in by their growing apices at the surface of the sponge, and the presence of which may greatly injure the quality of the sponge. The connecting fibres are only about 0.035 mm. in diameter, or even less, and the primaries are a little thicker, while the meshes between the fibres are so narrow as to permit of the soaking up of water by capillary attraction,

(After F. E. Schulze.)

Fig. 4.—Plakina monclopha.

a, Ciliated embryo (the central part should be shaded).

b, Part of section of ciliated embryo.

col, Inner cell-mass.

ec, External, columnar cells.

fl, Flagella.

c, Attached embryo, viewed from above, with the gastral cavity appearing in the interior.

d, Vertical section of attached embryo.

e, Rhagon stage, viewed as a transparent object, showing the inhalant pores on the surface and the flagellated chambers in the interior; the osculum is not shown.

f, Part of vertical section through adult sponge, showing the folded choanosomal lamella or spongophare.

ov, Ova. bl, Embryo.

(After F. E. Schulze. From a plate in Zeitschrift für Wissen. Zoologie, by permission of Wilhelm Engelmann.)

Fig. 5.—Plakina monclopha.

Spicules, a-e, tetracts or calthrops; f-k, triacts or triradiates; l-t, diacts, showing how the monaxon form (1) may be derived from the primitive tetract (a) by suppression of actines.

(After F. E. Schulze. From a coloured plate in Zeits. für Wissen. Zoologie, by permission of Wilhelm Engelmann.)

Fig. 6.—Euspongia officinalis (bath sponge). Part of vertical section showing general arrangement of skeleton and canal-system.

p.f, Primary fibre of skeleton.

s.f, Secondary fibres.

d.p, Dermal pores (inhalant).

i.c, Inhalant canals.

e.c, Exhalant canals.

f.c, Flagellated chambers.

(After F. E. Schulze. From Lankester's Treatise on Zoology.)

Fig. 7.—Euspongia officinalis (bath sponge). Skeleton. Fibre surrounded by spongoblasts.

sp.f, Spongin fibre; sp.bl, Spongoblasts. Coll, Collencytes.

(After F. E. Schulze.)

Fig. 8.—Euspongia officinalis (bath sponge). Diagram of the arrangement of the canal-system as seen in vertical sections of two young individuals.

d.p, Dermal pores; o, Oscula; r, Rock to which the sponges are attached.