120 M. K. A. Zittel on Fossil Lithistidce. 



the Lithistidse to study the canal-system in macerated or fossil 

 skeletons with as much certainty as in fresh specimens. 



Six different modifications of the water-circulation may be 

 distinguished in the Lithistidse : — 



1. A special canal-system is entirely deficient. 



2. From one or both surfaces, finer or coarser, arched and 

 frequently ramified canals penetrate, to a greater or less depth, 

 into the wall. 



3. Simple or branched, more or less curved canals run in a 

 nearly horizontal direction from without inwards, and termi- 

 nate in the stomachal cavity, whilst a second system of 

 similar radial canals traverses the wall in a centrifugal direc- 

 tion and opens at the surface. 



4. Simple, straight, often capillary radial canals traverse 

 the wall in a centrifugal direction from within outwards ; 

 besides these there is sometimes a second system of curved 

 canals running more or less parallel to the outer surface, and 

 opening into the stomachal cavity. 



5. The sponge-body is traversed by vertical tubes, to which 

 radial canals are frequently superadded. 



6. The whole wall consists more or less distinctly of per- 

 pendicular skeletal lamellae or wedge-shaped segments, 

 between which the water-circulation takes places in a radial 

 direction. 



The first and simplest case, that of the complete deficiency 

 of a true canal-system, occurs only in a few genera of globular, 

 disciform, or nodular form {Sjwngodiscus, Lecanella^ Platy- 

 chonia^ BoUdium^ Mastosta). In these the entire water- 

 circulation takes place solely through the larger or smaller 

 interspaces of the skeletal substance. On the surface there 

 are no large oscula ; and in these forms there is also never 

 a stomachal cavity : either the surface presents exactly the 

 same structure as all the rest of the sponge-body {Hpongo- 

 dtscus), or the substance of the skeleton becomes a little 

 condensed and leaves only fine roundish pores {BoUdium^ 

 Mastosia) . 



From this simplest arrangement we find all intermediate 

 steps to the second modification, in which the surface is covered 

 with larger or smaller orifices, from which more or less curved 

 canals penetrate into the interior of the wall. In the external 

 form of these Lithistidaa the cup, vase, basin, or laminar form 

 prevails. In certain genera [Chonella) the orifices are scarcely 

 \-\ millim. in diameter, like pores, and corresponding to this 

 the canals also are fine and but slightly developed. The 

 laminiform or cup-shaped sponge-bodies also therefore possess 

 no distinct stomachal cavities, unless the wide central space 



