30 Mr. H. J. Carter on the Anatomy 



system or plexus ; while the vents may exist here "and there 

 singly and separate or in groups, where their size and number 

 indicate those of the system with which they are respectively 

 connected. Moreover the sponge has the power of opening 

 them at one place and closing them at another ; while in 

 abnormal states their currents may even be reversed. The 

 notion that each vent represents one " person " or individual 

 sponge is not always correct, as I have shown in the young 

 Spongilla, wherein a second vent is occasionally produced, 

 thus forming two for its excretory canal-system ('Annals,' 

 1857, vol. xx. p. 31). Among the calcareous sponges, too, 

 Orantia compressa may have one, two, three, or more vents 

 to its cloacal cavity &c. One vent therefore does not always 

 represent one " person." 



Function of the Pore- and Vent-Systems respectively . 



As the function of the pore-system is to admit nourishment 

 to the interior of the sponge, so that of the vent- or excretory 

 canal-system is to carry off the refuse. Hence in sponges 

 growing horizontally, like the fungus Polyporus, the pores 

 are generally on the upper and the vents on the lower 

 surface ; but when sponges grow (as they usually do) on the 

 under surface of rocks, the mammiform or tubular prolonga- 

 tions are directed downwards and terminated by the vents. 

 Where the sponges are tubular, as before stated, the vents 

 open on the inner side of the tube, which has thence been 

 called by Dr. Bowerbank the " cloaca." But whether the 

 sponge be tubular, and thus provided with one great cloacal 

 opening, or whether flat and provided with several, each 

 kind of vent is but the termination of a cavity into which 

 many minor vents have previously opened ; so that the great 

 cloacal or general vent is but a modification of the smaller and 

 much more common kind. 



For a detailed description of the function of the pores and 

 vents, under the appellations of " afferent " and " efferent 

 canals," see my account and illustrations of the development 

 of Spongilla from the seed-like body, 'Annals,' 1857 (vol. xx. 

 p. 27 et seq.). But a special study of every thing connected 

 with the pore- and vent-systems respectively throughout the 

 Spongida is much to be desired ; for there is yet much to be 

 revealed concerning their functions. 



'f-i 



Flesh- Spicules. 

 As there is a class of spicules entirely connected with the 



