VII STRUCTURE OF £/^^yZ)^ 77^ 175 



to be successful if perpendicular timbers such as lock-gates are 

 examined, or the underside of floating logs or barges, or over- 

 hanging branches of trees which dip beneath the surface of the 

 water. 



The sponge is sessile and massive, seldom forming branches, 

 and is often to be found in great luxuriance of growth, masses of 

 many pounds weight having been taken off barges in the Thames. 

 The colour ranges from flesh-tint to green, according to the 

 exposure to light. This fact is dealt with in a most interesting 

 paper by Professor Lankester,^ who has shown not only that 

 the green colour is due to the presence of chlorophyll, but that 

 the colouring matter is contained in corpuscles similar to the 

 chlorophyll corpuscles of green plants, and, further, that the flesh- 

 coloured specimens contain colourless corpuscles, which, though 

 differing in shape from those which contain the green pigment, are 

 in all probability converted into these latter under the influence 

 of sufficient light. The corpuscles, both green and colourless, are 

 contained in amoeboid cells of the dermal layer ; ^ and in the same 

 cells but not in the corpuscles are to be found amyloid substances. 



The anatomy of Uphydatia fluviatUis is very similar to that 

 of Halichondria panicea, differing only in one or two points • of 

 importance. The ectosome is an aspiculous membrane of dermal 

 tissue covering the whole exterior of the sponge and forming the 

 roof of a continuous subdermal space. This dermal membrane 

 is perforated by innumerable ostia, and is supported above the 

 subdermal cavity by means of skeletal strands, which traverse 

 the subdermal cavity and raise the dermal membrane into tent- 

 like elevations, termed conuli. The inhalant canals which arise 

 from the floor of the subdermal cavity are as irregular as in 

 jff. panicea, and interdigitate with equally irregular exhalant 

 canals ; these latter communicate with the oscular tubes. Be- 

 tween the two sets of canals are the thin folds of the choanosome 

 with its small subspherical chambers provided with widely open 

 apopyles (Fig. 70). The soft parts are supported on a siliceous 

 skeleton of oxeas, which may have a quite smooth surface or may 



1 Quart. Journ. Micr. Sci. xxii. 1882, p. 229. 



2 But see Gamble and Keeble, Quart. Journ. Micr. Sci. xlvii. 1904, p. 363, who 

 sliow that various green animals really owe their colour to "algae," though the 

 infection with the "alga " is difficult to detect because it takes place by means of a 

 colourless cell. See also Zoochlorella, on p. 126. 



