418 STRUCTURE AND ECONOMY OF TETHEA. 



internal development of the mass ; so that the efferent chan- 

 nels of this sponge are in fact mechanical openings — vents 

 produced by its internal growth. 



I have already described the textures which enter into 

 the structure of the rind, and which line its water-tubes. 



I have also stated that the same textures surround the 

 afferent canals. 



The perforated membrane of the grooves contains more abun- 

 dant fibrous texture, and a greater number of stellate spicula. 



The walls of the afferent water-canals in the intermediate 

 masses are lined by minute bodies, which may be denominated 

 sponge-particles. My specimens having been preserved in 

 alcohol, I have been unable to detect on the sponge-particles 

 the cilia which they present in the living Grantia,* and 

 for the discovery of which we are indebted to Dr. Dobie and 

 Mr. Bowerbank. I have no doubt, however, that these par- 

 ticles are covered with cilia, and that the currents through the 

 canals are produced by their agency. The particles present- 

 the usual nucleus, cellular contents, and the clear space or 

 contractile vesicle. They are from the 2000th to the 3000th 

 of an inch in diameter. 



In the intermediate masses of this sponge there are also 

 to be met with very abundantly, but irregularly distributed, 

 minute cellules about the 4000th of an inch diameter, many 

 of them with tails or processes like spermatozoa, and occa- 

 sionally exactly resembling spermatozoa. From a preserved 

 specimen I cannot pretend to determine whether these 

 minute bodies are the earlier forms of other sponge-particles, 

 or whether they are actually spermatozoa. 



Ova are also very abundant in the intermediate masses. 

 When minute the ova exhibit a nucleus or germinal vesicle, 

 but when larger they present the appearance of mulberries, 

 or minute spherical botryoidal masses. 



* See Goodsir's Annals of Anatomy and Physiology, pp. 127-129. — Ens. 



