ORGANS OF DIGESTION. 311 



and for some time after their development in a fixed condi- 

 tion has commenced, present no perceptible canals or cavi- 

 ties of any kind in their body 5 nor do the polypipherous 

 animals while they continue in the same free state of ciliated 

 moving gemmules. As the development of the porifera 

 proceeds, minute openings are observed to form on the sur- 

 face, which extend gradually through the body, producing 

 internal canals which terminate superficially in vents or fecal 

 orifices. From the incessant currents conveyed through the 

 body of these animals, it would appear that all parts of their 

 interior, like the exterior surface, of their general cellular 

 tissue, are adapted to admit by endosmose, and to assimilate 

 nutritious matter to the texture of their body. On watching 

 the streams of water that issue from the vents, minute floc- 

 culent particles are observed incessantly detached from the 

 interior, and thrown out with the currents, these appear to 

 be fine mucous pellicles excreted from the surface of the 

 internal canals, as the residue of digestion thus detached 

 from the body. A similar mode of excretion is often seen 

 on the naked mucous surface of zoophytes, where thin 

 pellicles are -periodically detached from the soft exterior of 

 the body. 



In the spreading sessile species of porifera, as in the halina 

 papillaris (Fig. 108.) so common on all parts of our coasts, both 

 the small absorbent pores (108. , a, a.) and the larger fecal 

 vents (108. b, b.) are necessarily disposed on the same general 

 external free surface, the inferior surface (108. g.} being fixed 

 to the rocks or other sub-marine bodies and thereby com- 

 pletely closed. The upper free portion of the body, as in 

 most other animals, is more appropriated to nutrition, and the 

 lower or posterior part to generation. A vertical section of 

 the body, shows the continuation of the pores (108. d.) which 

 lead to the larger branches, canals (108. e.) and vents (108. 

 b, b.). Along with the small portions of feculent matter 

 (108. b. b.) are seen propelled from the vents numerous 

 ovate reproductive gemmules (108. c. c.) after they have 

 been developed in clusters (108. /.) in the deeper parts of 

 the body, and have escaped into the internal canals (108. 

 /*/*) In the tubular species, as in the leuconia com- 

 pressa (Fig. 3.), the whole outer surface or periphery of the 

 body is appropriated exclusively to the absorbent pores 



