182 ANATOMY AND PHYSIOLOGY 



"TfiTHiE {Tethea). 



" Polypier tubereux, subglobuleux, tres fibreux interieure- 

 ment; k fibre subfasciculees, divergentes ou rayon- 

 nantes de rinterieur a la circonference et agglutinees 

 entre elles par un peu de pulpe ; a cellule dans un en- 

 croutement cortical quelquefois caduc. Les oscules 

 rarement perceptibles." 



Dr. Johnston's version of the generic characters differs 

 slightly from Lamarck's. They are as follows : — 



" Sponge tuberous, suborbicular, solid and compact, invested 

 with a distinct rind or skin, the interior sarcoid loaded 

 with crystalline spicula collected into bundles and 

 radiating from a more compact nucleus to the circum- 

 ference. Marine." 



It is much easier to find faults in the generic characters 

 of both the authors quoted, than it is to improve them. 

 The extreme simplicity of the structural characters of Tethea 

 is a strong temptation to endeavour to multiply them ; but 

 in doing so. Dr. Johnston has introduced two — the struc- 

 ture of the dermal portion of the sponges, and the tuberous 

 nature of its surface — which are not common to all the 

 known species. If we consider the word " tuberous " in 

 the usual English acceptation of the word, as a body " full 

 of knobs or swellings," then, very few or perhaps none of 

 the species of Tethea would, in their natural condition, 

 exhibit this character, but all of- them would be in a greater 

 or less degree subglobular. Dr, Johnstone's description of 

 Tethea was founded on the structure of T. lyncurium only, 

 and in this species the "thick rind " is very distinctly to 

 be seen, but in other species this structure is totally 

 wanting. It therefore ceases to be of value as a generic 

 character, and becomes a specific one only. Under these 

 circumstances I propose the following modification of the 

 previously published generic characters : — 



