78 SYNOPTIC COLLECTION. 



face of the ectoderm is differentiated into a hardened 

 wall or cortex with a distinct fibrous layer. The skeleton 

 in Tethya has a radiate arrangement. The spicules 

 when typical have a long straight axis with three curved 

 horns. Besides these there are straight spicules with 

 both ends alike, and also star-like silicious forms. Fig. 2 

 is a vertical section of Tethya showing its system of 

 tubes, the cloacal opening to one side, and the silicious 

 threads extending from the base. The power of adapta- 

 tion possessed by most animals in a greater or less degree 

 is strikingly seen in Tethya. With a rounded form and 

 a yellowish color which have given it the name of "the 

 orange of the sea" (see fig. i), it has succeeded in secur- 

 ing a firmer hold by means of long, tough, silicious 

 threads (PI. 74) which act as anchors penetrating the 

 mud and holding the growing sponge upright. The spe- 

 cies shown has a peculiar adaptation of this habit, having 

 a network of silicious threads like a mat of coarse wool 

 on its base. These catch the fine gravel sifted out of the 

 mud by the movements of the animal caused by the 

 waves, and this gravel makes its lower side much the 

 heavier. If now the animal is upset or swept away by 

 the current or waves, the gravel acting as ballast will 

 always serve to keep it right side up. 



One of the most complex forms of this group is Geodia 

 (No. 75). Here the outer part is differentiated to form 

 a cortex and the mesoderm is thick. The spicules are 

 unusually large and can be seen with the naked eye. 



SlLICEA. MONAXONIA. 



There is no sharp line of division between the Tetrac- 

 tinellida and the Monaxonia. Suberites (No. 76 ; No. 77, 

 dried specimen) is instructive, since it has adapted itself 

 to a free life on shifting sands. Its pores are so small 

 and the structure so dense that the sand cannot pass 



