LITHISTID SPONGE FROM THE DEEP SEA. 321 



have their bases sloping gradually on to the body. The oscular 

 openings are unequal in size, are crateriform, and their wall is 

 evidently stout. (Plate XXIV. fig. 1.) 



A transverse section of the body reveals a thin dense cortical 

 part, enclosing a lax sponge-like internal structure, with cavities 

 and canals of different sizes ; and these end externally in the 

 oscular projections. (Plate XXIV. fig. 2.) 



The cortical part exhibits a totally different structure to that 

 within, and is dense and close ; it forms the outside of the body 

 and of the oscular processes. No pores are visible on the outside 

 of the body witli the naked eye ; but the microscope reveals minute 

 spaces between the margins of the foliato-expando-ternate spicula 

 of the derm. Water enters very readily through the dense cortex 

 into the interior. 



A section through the sponge-body, made sufficiently thin to be 

 used under transmitted light, without disturbing the position of 

 the structural elements, revealed the histology ; and a thin slice 

 made parallel with the surface enabled its details to be satisfac- 

 torily described and figured. 



The transverse section shows the dark line of the dense but 

 thin cortex on the outside, and within it concentric layers of 

 spicula grouped in different manners, the outer layer being con- 

 nected with the connective spicula of the cortex by means of 

 short, stout, cylindrical processes. No free stems project inwards 

 from the foliated edged peltate-looking, connective spicula just 

 noticed ; for the stems are invariably attached to the proper spi- 

 cula of the body just within. These outer proper spicnlaofthe 

 body are in very close order, and within them is a series of radial 

 spicula massed into radial columus with spaces between them, 

 giving the appearance of rather a lax tissue. The columns im- 

 pinge, internally, on a thick concentric layer of shorter spicula 

 much confused ; and this is attached within to a columnar layer 

 like that external to it. This alternation of close and open 

 concentric layers is repeated several times with more or less 

 regularity. 



The result of this arrangement and grouping of spicula is to 

 develope a series of moderately sized spaces in concentric series, 

 and to establish a less permeable set of layers between them. 

 Here and there the spaces become larger, and in some places 

 small cul-de-sacs or tubes are formed ; but it is done at the 

 expense of the concentric close layers which abort ; and the long 



