THE SPONGES. 9 



variation in the number of spicules which combine to form the side of a 

 skeletal mesh, p. 145. 



S. Uiicorr elated variation in the mcgascleres of an mdividual, i. e. variation 

 apparently not correlated with the structural peculiarities of the body- 

 locality. 



As regards size of the spicules, there seems to be noticeable variation 



in all sponge species. The shape of one of the macramphidiscs varies 



considerably in Hi/alonema hiunchoratiim, p. 20. In Caulopliaciis schiilzei 



the principal hexacts are occasionally tuberculated, p. 40, and in the same 



species the tuberculation of the pentacts varies, p. 43. The character of 



the diact ends varies commonly, e.(/. in Bathf/dorus levis spinosus, p. 49. 



In Farrea mexicana the pentacts vary in respect to tuberculation, p. 66. 



In Eurete eredum the character of the distal ray of the gastral 



pinnies varies extensively, p. 62 (comp. F. E. Schulze, 1899). In 



Petrosia variabilis crassa the oxea sometimes assumes the shape o£ a stron- 

 gyle, or style, p. 115. In Fenares foliaformis the triaenes vary con- 

 siderably, p. 109. In Fachi/chalina acapulcensis the size of the oxea 



varies within wide limits, p. 121. In Phakellia lamelligera the oxeas 



and the two kinds of styles all vary considerably in shape, p. 152. 



4- Uncorrelated variation in the microscleres of an individual. 



The microscleres as well as the megascleres very commonly vary in size 

 and detailed shape, although in them, as in the megascleres, there is a 

 size and pattern which are characteristic of the individual (species), i, e. to 

 which the majority of the spicules of an individual conform. 



Some striking cases of variation are afforded by the micro-oxyhexacts 

 of Hi/alonema hianchoratum, p. 19, plesiasters and spirasters of Thenea fen- 

 esirata, p. 86, sigmata of Ti/lodesma alba, p. 133. 



5. Correlated variation in the spicides. 



In some cases the variation of spicules is obviously not ungoverned by 

 the rest of the body, but is correlated with body-locality. 



Thus while the pentacts in Hexactinella labijrintUca vary at large in 

 respect to length of the several rays, the pentacts overlying the larger 

 inhalent canals commonly have noticeably short proximal rays. Such a 

 phenomenon would customarily be referred to as " adaptive." The vari- 

 ation of dichotriaenes toward the protriaene type, round the pore areas 

 of Thenea, pp. 85, 89, 92, 95, is another instance of the same phenom- 

 enon. A complex instance of correlated variation is aflbrded by the 



