One genus, _Hexadella_, has been regarded by its discoverer Topsent[229] as an Hexactinellid. The same authority places _Oscarella_ with the Tetractinellida; it is more difficult to suggest the direction in which we are to seek the relations of the remaining type, _Halisarca_. _Hexadella_, from the coast of France, is a remarkable little rose-coloured or bright yellow sponge, with large sac-like flagellated chambers and a very lacunar ectosome. _Oscarella_ is a brightly coloured sponge, with a characteristic velvety surface; it is a British genus, but by no means confined to our shores. Its canal system has been described by some authors as diplodal, by others as eurypylous. Topsent[230] has shown, and we can confirm his statement, that though the chambers have usually the narrow afferent and efferent ductules of a diplodal system, yet since each one may communicate with two or three canals, the canal system cannot be described as diplodal. The hypophare attains a great development, and in it the generative products mature. The pinacocytes, like those of Plakinidae, and perhaps of _Aplysilla_, are flagellated. _Halisarca_, also British, is easily distinguished from _Oscarella_ by the presence of a mucus-like secretion which oozes from it, and by the absence of the bright coloration characteristic of _Oscarella_. It naturally suggests itself that the coloration in the one case and the secretion in the other are protective, and in this respect perform one of the functions of the skeleton of other sponges. The chambers are long, tubular, and branched. There is no hypophare. {197}CLASS II. HEXACTINELLIDA[231] _Silicispongiae, defined by their spicules, of which the rays lie along three rectangular axes. The canal system is simple, with thimble-shaped chambers. The body-wall is divided into endosome, ectosome, and choanosome._