98 ANATOMY AND PHYSIOLOGY 



and convey it to the inmost depths of the sponge, ramifying 

 continually like arteries as they proceed in their course 

 downward, until they terminate in numerous minute 

 branches. The inhaled fluid is then taken up by the 

 minute commencements 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 their 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. 

 Tn 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 sub- 

 stance 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 find 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 fgecal matter that issues 

 may be discharged free of the surrounding membraue. 



The large fistular projections which form such striking 

 and beautiful objects in the genus Alcyoncellum are also 

 great cloacal organs, their dermal membranes abounding in 

 pores, and their inner surfaces furnished with oscular orifices, 



