SPONGES. 257 



each provided with its individual system of radiating channels, 

 which, however, gradually merge into the prevailing radiate ar- 

 rangement. The depressed central portion of each osculum is 

 occupied by the apertures of a limited number of rather large 

 thin-walled vertical tubes, which traverse the sponge wall to 

 half or less than half its thickness. Those in the central oscu- 

 luni extend throughout the stem. 



Interior skeleton composed of } { shaped spicules, arranged in 

 linear series so as to leave minute canals of triangular, quad- 

 rate, or polygonal form. Just below the centre of a vertical 

 section of the sponge wall, these capillary canals run nearly 

 parallel with the lower margin. From this region they proceed 

 in a gradually diverging and curved direction to each surface.* 



Both surfaces sometimes covered with a dense dermal layer. 



Types, A. florifera and A. mammulata. 



The minute internal structure of the species referred to this 

 genus and described on the following pages, is practically iden- 

 tical in all. This we believe to be largely due to the destruc- 

 tion of the finer details of structure, consequent to the replace- 

 ment of the original siliceous material by crystalline calcite. 

 The same may be said of the spicular mesh of Zittelella, Edrio- 

 spongia and Streptosolen when compared with that of Anthas- 

 pirtella. We were, therefore, obliged to depend entirely upon 

 external features and variations in the systems in distinguish- 

 ing the genera and the species referred to each. As these char- 

 acters are often unreliable among the sponges, we will not be 

 surprised if future investigations prove that we have made 

 more species and, perhaps, even genera than was necessary. 

 The field being comparatively new and exceedingly difficult, we 

 expect our work to prove faulty in some respects; yet, what- 

 ever errors we may have fallen into, we will find consolation in 

 the conviction that we have done the best we could under the 

 circumstances.! 



* For further information respecting the interior see discussion on the Anthispidel- 

 lidae, on pages 221 to 226, 



t Since the above wa- written Mr. Ulrich visited Dixon and succeeded in collecting 

 a considerable number of these sponges. Their study proved very gratifying to us, 

 since we found no difficulty whatever in placing every specimen save a few, belonging 

 to as yet undescribed species. 



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