786 
DK. J. S. BOWEEBAI^K ON THE AI^ATOMT 
The incurrent series have their origin in the intermarginal cavities immediately within 
the dermal membrane, and their large open mouths receive from these organs the water 
inhaled through the pores and convey it to the inmost depths of the sponge, rami^ing 
continually like arteries as they proceed in thek course downward until they terminate 
in numerous minute branches. The inhaled fluid is then taken up by the minute com- 
mencements of the excurrent series, which continually unite as they progress towards 
the surface of the sponge, in the manner of veins in the higher animals, until they 
terminate in one or more large canals which discharge then- contents through the 
oscula of the sponge. This system is found to obtain in the whole of the genus Spongia 
and in the massive Halichondroid sponges, which have their oscula dispersed over their 
external surfaces. By this mode of organization the inhaled fluid, laden with nutritive 
particles, is poured at pleasure into the internal cavities of the sponge, flowing over 
extensive membranous surfaces coated with sarcode; so that the aggregated surfaces 
become a great system of intestinal action, fully equal in proportional extent to that 
of the intestines of the most elaborately organized mammal. 
They do not in every genus exhibit the regularity of structure described above, and in 
some cases the canalicular form resolves itself into a series of irregularly formed spaces. 
In other cases, where a common cloaca exists, there appears to be but one system of 
interstitial canals, those which convey the inhaled fluid from the pores through the 
substance of the sponge to the parietes of the great central cloacal cavity which 
receives the whole of the faecal streams, rendering the system of excurrent canals 
unnecessary. 
In the Cyathiform sponges we And a somewhat similar structure. The outer portion 
of the cup is essentially the inhalant surface and the interior of it the exhalant one, and 
there accordingly we generally find a great number of small oscula dispersed on all 
parts of it, very often having their margins slightly elevated, that the fsecal matter that 
issues may be discharged free of the surrounding membrane. 
The large fistular projections which form such striking and beautiful objects in the 
genus Alcyoncellum are also great cloacal organs, their dermal membranes abounding hi 
pores, and their inner surface furnished with oscular orifices, the intervening space being 
occupied by the interstitial cavities, the interior forming one large cloacal cavity, which 
discharges its contents through a cribriform mouth at its distal end. In Grantia both 
systems, the incurrent and excurrent interstitial canals, become very nearly obsolete, the 
large intermarginal cavities or cells imbibing the water through their pores on the distal 
extremities, and becoming enlarged and elongated until they reach the parietes of the 
great central cloaca, into which they discharge their contents, each through a single 
osculum, into a short depression or cavity in the parietes of the great cloaca, and this 
shallow cavity represents the nearly obsolete system of excurrent canals. 
The membranes lining the incurrent and excurrent canals are frequently highly 
organized. In the common honeycomb sponge of commerce, when in the same condi- 
tion as when taken from the sea, these canals are constructed of a series of compound 
