SPONGES.-HALLMANN. 121 



Megascleres. — It is difficult to say whether these belong to 

 one, two, or three categories. At hrst sight it might appear 

 that two kinds are to be distinguished, viz., (i.) stouter tylo- 

 styli, with a tylosis of only moderate size, which is perhaps 

 most frequently subterminal in position and more or less re- 

 duced (or sometimes absent) and which has a quite smooth 

 surface and not very variable contour; and (ii.) very slender 

 tylostyli (of scarcely lesser length), in which the frequently 

 relatively large tylosis is extremely variable in shape and size, 

 often exhibits tubercular irregularities of the surface, and is 

 occasionally several times repeated. The former spicules vary 

 in length from less than 200 ;t to slightly more than 700 ;t, 

 and in diameter up to u /< ; it is seldom, however, that their 

 length exceeds 650 }i or their diameter 10 fi; usually they 

 taper to a quite sharp point, but more or less strongylote 

 terminations occur. The latter or slender tylostyli appear 

 to be rarely more than 3 }i in diameter and may attain a length 

 of at least 640 ^. A more thorough scrutiny of the spicules 

 shows, however, that forms intermediate between these kinds 

 occur, though they are comparatively few in number; and, 

 taking all the facts into consideration, it is not possible to 

 decide with certainty whether the two degrees of stoutness are 

 merely the expression of different developmental stages, or 

 whether they have a more important significance and indicate 

 either that the spicules have undergone a partial differentia- 

 tion into two groups or actually represent two distinct orders. 

 Again, it is to be observed that the stouter spicules vary con- 

 siderably in length ; and on analysis it appears not improbable 

 that in another respect also a certain amount of differentiation 

 into two groups has been affected since a relatively large pro- 

 portion of the spicules below a length of 300 ;/ or thereabouts 

 (the diameter of which is 6 to 8 ;/) are simply stylote, and are 

 subfusiform in shape. But these peculiarities are not confined 

 to the shorter spicules, and are not, indeed, strikingly charac- 

 teristic of them. These shorter spicules appear to greatly 

 predominate amongst those which stand perpendicularly at 

 the surface of the sponge. 



Microscleres. — Spirasters of two kinds occur: 



(i.) Small tuberculated forms, extremely variable in shape, 

 often with the tubercules chiefly confined to one side, 

 12 to 32 fi in length, and up to 4 ji in breadth exclu- 

 sive of the tubercules. These, together with an 

 inconsiderable admixture of those of the second kind, 

 form an exceedingly thin but apparently not continu- 

 ous superficial layer, and also occur in limited 

 numbers in the canal walls. 



