35io THE SPONGES 



THE CALCAREOUS SPONGES Class Calcarea 



In this group the skeleton is formed of spicules of carbonate of lime, shaped 

 like three-rayed stars, four-rayed stars, or needles. The triradiate occurs most fre- 

 quently in its typical form, the three rays being equal, in one plane, and forming 

 an angle of 120 with each other. The spicules are transparent and glassy when 

 viewed separately, but white and opaque in mass. On placing a calcareous sponge 

 in acid, the skeleton dissolves away with effervescence. The class is divided into 

 two groups, namely, the Homoccela and the Heterocoela, in the former of which the 

 collar cells line the whole of the interior of the simple or branched sac; while in the 

 latter they are confined to the radial tubes, or the ends of branched radial tubes or 

 canals, the gastric cavity with a part of the canal system being lined with flat cells. 

 The simple ascon sponge (Ascetta primordialis} forms a minute stalked sac open 

 above, and with its thin walls perforated by pores and supported by triradiate spicules; 

 the whole interior being lined with collar cells. The simple sac may give off a stolon, 

 whence arise other sacs, or it may branch, and the branches again divide forming a 

 tree-like growth; or, lastly, the branches may join together and form a complicated 

 meshwork of sacs or tubes. The allied Leucosolenia, shown in the illustration, is a 

 branched ascon found on seaweeds in the form of clusters of small white tubes. 



The ciliated sycon {Sycandra) and Grantia are common among seaweeds and 

 in rock crevices on the British coasts; both sponges are sycons, i. e. with tubes 



radiating out horizontally from a central cavity, and in 

 both are present the three kinds of calcareous spicules 

 (needles, three-rayed, and four-rayed forms). The walls 

 of the ciliated sycon are made up of closely - packed 

 tubes, lined with collar cells and opening into the vertical 

 gastric cavity. Rows of three-rayed spicules, arranged 

 in regular series, support the walls of the tubes, the 

 blind ends of which are protected by tufts of needles; 

 a layer of four-rayed spicules lines the walls of the 

 central cavity, the fourth rays projecting inward 

 and upw T ard into the cavity so as to form a wall 



A CALCAREOUS LKUCON f S P ikeS " Wh the ^^ 1S ^ W ' m % ^^ the 



SPONGE, Leucandra. interior the crown of needle bristles round the oscule 

 (Natural size.) is expanded, but when the sponge is inactive the bristles 



fall together and cover the oscule. Grantia lives on sea- 

 weeds or hanging down from rocks, and several specimens are usually found 

 together. They resemble small white leaves, or flat bags, averaging about an inch 

 in length and one and a half in width, though sometimes much larger. When 

 quite young and small they possess only one oscule, but larger specimens possess 

 several on the thin margins. When the sponge is active, the flat leaf fills out like 

 a small paper bag. The leucons usually possess tubular or hollow knob-shaped 

 bodies with thick walls, in which ramify a double system of canals, in-current 

 from the surface to the flagellated chambers, and out-current from the latter to the 

 gastric cavity. 



