CALCAREOUS GROUP. 



535 



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 Homocoela 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 inwards 

 and upwards into the cavity so as to form a wall 

 of spikes. When the current is flowing from the 

 interior the crown of needle bristles round the oscule 

 is expanded, but when the sponge is inactive the 



bristles fall together and cover the oscule. Grantia lives on seaweeds 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. 



On making a section or teasing a fragment of a calcareous sponge in the 

 spring, the minute embryos will often be seen. When the embryo leaves the 

 parent sponge, it consists of an extremely minute oval cyst or vesicle (v) with a 

 small central cavity, and is formed of two kinds of cells. The anterior half, or the 

 part in front when the embryo is swimming, is composed of a number of long 



A CALCAREOUS LEUCON SPONGE, 



Leucandra (uat. size). 



