22 THE VOYAGE OF H.M.S. CHALLENGER. 



diciilar plate-like form {Chonelasma, Pis. LXXXVII. -XCL). The wall of the cup may 

 be complicated by thimble-like sacculations, as in Aphrocallistes hocagei (PI. LXXXIII. 

 fig. 1), and, in such cases, the gastral cavity may be separated by several transverse 

 net-like diaphragms into a series of partitions. If the outer margin of a stalked, 

 originally cup-shaped sponge, becomes folded outwards and downwards through great 

 development of the median portion, a fungoid form arises which, in the genus Caulo- 

 phacus (Pis. XXIV.-XXVL), exhibits several varieties of outline. In this way then, as 

 the gastral cavity and osculum have thus been lost, what was originally the internal 

 gastral has become the upper and outer surface, so that the water enters the body 

 from below, and escapes again from the upper outer wall. In Aulochone (Pis. LXVL, 

 LXVIIL) the originally upper portion of the gastral membrane has, on account of the 

 folding of the oscular waU, been turned towards the outside, and thus forms the outer 

 wa.ll of the cylindrical or approximately hemispherical body, while the under portion 

 of the gastral cavity along with the lumen of the tube-like stalk connected with it, has 

 remained unchanged. 



In many Dictyonina the elongation of the sac-like body, without any marked 

 thickening of the waU, results in the formation of more or less thin-walled tubes 

 in which the lumen remains approximately the same. These tubes often branch 

 in a tree-like fashion, frequently dichotomously [Aphrocallistes ramosus, PI. LXXXVI. 

 fig. 1), while the multiplication and union of branches may form an anastomosing network 

 of tubes, from which numerous terminal branches arise, each provided with an osculum. 

 The latter is the sta,te of the case, e.g., in the genera Fan-ea (Pis. LXXL, LXXII.) and 

 Eurete (Pis. LXXVII.-LXXIX.). In Farrea the young tube-wall begins on the outer- 

 most terminal branches as a very thin plate with a simply folded chamber layer, and 

 the whole wall is gradually somewhat thickened, with the increasing folding of the 

 chamber layer ; while in Eurete the ends of the tubes are continuous outgrowths of 

 the entire thickness of the wall. 



The main tubes in expanding into a funnel-shape sometimes give ofi" lateral branch- 

 tubes, which have a tendency to branch and anastomose, as in Peripliragella (PI. LXXX.) 

 and Aulocalyx (PI. LX.). In some species, which consist, for the most part, of a net- 

 like system of anastomosing tubes, with terminal and lateral oscular openings, a special 

 covering layer may occur which envelops the whole body, and which, as an independent 

 plate, not only spreads out laterally from the oscular walls as a fine porous skin or 

 net-like sieve for the inflowing water, but also extends over the oscula as a sieve plate, 

 with wider apertures. This remarkable structure, which may be termed a cover, is 

 seen in Aulocystis (PL CIV.), and also, though in quite diiferent form, in Semperella 

 (Pis. LL, LIL), where the oscula appear, not so much as round apertures, but rather as 

 irregular longitudinal clefts on the sides of the body. The covers of these genera difi"er 

 also in this : in Aulocystis the cover appears as a direct continuation of the entire wall 



