REPORT ON THE DEEP-SEA KERATOSA. 



21 



traverse the body of these Keratosa in all directions, and replace mechanically the absent 

 scaffold of stout spongin main fibres. This peculiar adaptation is found in sixteen 

 species, viz., in all the Stannomidse (nine species), in all the Spongelidse of the deep sea 

 (five species), and in two species of Psamminidae ; the symbiotic Hydroids are wanting in 

 most of the smaller forms, in the remaining five species of Psamminidae, and in all the 

 Ammoconidse (five species). A full description of the various symbiotic Hydroids is 

 given in the Appendix. Their chitinous tubes are usually filled by a dark brown mass, 

 which former observers supposed to be the phseodium of a large Ehizopod. Closer 

 examination proved it to be the ccenenchyma, which is usually more or less destroyed, 

 and showed also the gonangia and the hydranths. 



