ART. 1-' HEXACTINELLID SPONGES OKADA 33 



Form B (fig. 4, <^, /) appears chiefly in the gastral Layer, occasion- 

 ally intermixing with form A, arranged obliquely to the dermal sur- 

 face. It is somewhat smaller and shorter, about 400/a to 450/a long. 

 Its most striking characters are the shape of the terminal sj)ines pro- 

 jecting from the periphery of the terminal disk and the shape of the 

 lateral spines covering the proximal surface of the shaft. The ter- 

 minal disk of this form is usually 2t)[x to SOfx broad, and its summit 

 shows a generally hemispherically arched umbel, being raised in the 

 center into a weak external projecting boss or rarely a simply smooth 

 convexed surface. The spines on the periphery of the terminal disk 

 are usually much shorter and nearly straight, not so curved as those 

 of the preceding type. They are 30/* to 50/x. long and 36/x to 75/i 

 broad at the distal extension. The lateral spines on the proximal 

 surface of the shaft are also much shorter; projecting perpendicu- 

 larly at first, they are distally curved downward, instead of simply 

 projecting obliquely as those of form A. 



The uncinate is arranged perpendicularly or obliquely to the der- 

 mal surface. It usually penetrates the whole thickness of the body 

 wall. It is acerate and nearly straight; the outer half, nearer the 

 dermal surface, is alwaj^s much thicker than the inner, narrowed and 

 sharply jDointed end, and is spined at short intervals throughout. 

 These spines, which are all bent backward, are very short, slender, 

 and smooth. They are supported by small, weak, bracketlike proc- 

 esses arranged around the shaft in a spiral. The uncinate measures 

 about 1.3 mm long and exhibits some variability in length, though 

 this depends to some extent on the age of the individual spicule and 

 on the region of the sponge in which it occurs. 



Discohexasters of this species are probably found in two forms. 

 The common one (fig. 4, A), present in large numbers everywhere in 

 the sponge, is 60/a to SOju, in diameter and is provided with rather 

 strong principals crossing one another at the center, measuring IG/x 

 in length. From these arise 6 to 8 thin, nearly straight, and wealdy 

 divergent terminals 20/*. long and terminating with pinheadlike 

 disks at the ends. 



The other form (fig. 4:, g) may be present occasionally in the paren- 

 chyme, intermingled with the former. It is somewhat smaller in 

 size, measuring about 55/jl in diameter. Each short principal is sup- 

 plied with a bell-shaped, outwardly extending tuft of four to six 

 terminals, differentiating it from the preceding. These are somewhat 

 stronger, thicker, and fewer than those of the former form, and each 

 terminates in a small circular pinheadlike disk. 



The oxyhexaster (fig. 4, ^) occurs more frequently than the disco- 

 hexaster above mentioned, appearing chiefly on the dermal membrane 

 and sparsely in the parenchyme. It measures on the average about 

 118040—32 3 



