AND PHTSIOLOaT OF THE SPONGIAD^. 
789 
The progressive development of these inhalant areas, formed by combinations of the 
radii of the temate forms of spicula, in different species of sponges is very interesting. 
In Pachymatisma they are so indefinite that they can scarcely be said to exist. The 
temate spicula are few in number and very irregular in their mode of disposition, and 
a faint indication only of their future regular combination to form the dermal reticula- 
tion is apparent. In the more highly organized genus Geodia we find them in different 
species in progressive stages of combination, until, in G. IPAndrewii and Barretti, the 
apices of the radii of the temate spicula are interlaced with each other, and a conti- 
nuous irregular network is formed, each area of which is filled with the proximal termi- 
nation of an intermarginal cavity. In Bactylocalyx Pmttii, Boweebank, MS., the 
stmcture advances another stage towards perfection. 
There is the same design as that exhibited in the construction of the dermal areas in 
Geodia IPAndrefwii and Barretti, but there is a considerable difference in the applica- 
tion of the areas produced by the combinations of the temate apices. In Geodia these 
areas are placed beneath the highly organized and regularly formed intermarginal cavi- 
ties, and form the framework and support of their valvular proximal ends ; while in 
Bactylocalyx Prattii they are situated above the distal ends of the intermarginal cavi- 
ties of the sponge, which have not the regular structure and valvular appendage of 
those of Geodia, but are similar to the like organs in the Halichondroid sponges, and 
in this position they serve only to support and strengthen the dermal membrane, which 
adheres firmly to their distal surfaces. In this situation they are subject to a greater 
chance of pressure and disruption than the more deeply seated ones of Geodia, and 
accordingly we find extra provisions for the safety of the junctions of their radii. The 
shafts of these spicula are short, stout, and conical, and they penetrate but a very short 
distance into the substance of the sponge. They do not appear to be cemented to any 
part of the rigid siliceo-fibrous skeleton, but are merely plunged into a somewhat thick 
stratum of membranous structure reposing on the surface of the skeleton. Their radii 
are compressed considerably and extended laterally, so that their planes are in accord- 
ance with that of the dermal membrane, and they present a greater amount of adhesive 
surface than those having cylindrical radii. The temate rays ramify irregularly. Some- 
times one ramus, after slightly pullulating, remains nearly obsolete, causing the branch 
to assume a geniculated form like some of the ramifications of a Deer’s horn, and no 
two appear to be exactly alike ; in fact there is every appearance that each ray is 
influenced and modified in its development by the necessities of combination with the 
adjoining spicula, and their apices are directed in such a manner that they lap over each 
other in opposing lines, so that each two form a spliced joint, giving a much greater 
amount of strength than the mere crossing of the radii at various angles as in those 
of Geodia. The inhalant areas thus formed appear to differ very slightly from those 
of HalicTiondria jyanicea, in each of which several pores are opened, while those 
of Bactylocalyx Prattii seem to be devoted each to a smaller number (Plate XXIX. 
figs. 8 & 9). 
5 p 2 
