336 HYALONEMA (OONEMA) CRASSIPINULUM. 



In the axial part of the sponge a few much larger amphioxes were observed; 

 they had a maximum length of 8 mm. and were 160 /^ thick. These appear to 

 take part in the formation of the upper end-part of the stalk, which is imbedded 

 in the body of the sponge. 



The spicules of the stalk are broken off rather short. Where they arise 

 from the body of the sponge they are 50-500 ^ thick. 



The acanthophores in the sponge-body (Plate 94, figs. 24-33) are mostly diac- 

 tines and tetractines, but monactine, triactine, and pentactine forms also occur. 

 The monactines are tylostyle, the diactines centrotyle. The monactine and 

 diactine rhabd-forms are 160-840 yu long, and 13-29 /j. thick near the tyle. 

 The tyle in the longer ones is often very large. The tri- to pentactines are 

 85-480 fi, on an average 223.6 /i, in maximum diameter, and have rays 12-30 ^ 

 thick at the base. The ends of the fully developed rays are always spiny. 

 The same is the case in the rays reduced in length, provided the reduction has 

 not gone too far. The rays reduced to mere knobs are smooth. The central 

 parts of these spicules are usually smooth (Plate 94, figs. 24, 26, 28-33), more 

 rarely covered with sparse small spines (Plate 94, figs. 25, 27). 



The acanthophores of the sponge used by the Palythoa to build its skeleton 

 (Plate 94, figs. 14-23) are di- to pentactine. The diactines are not nearly so 

 numerous among them as among the basal spicules from the sponge. The 

 diactines are centrotyle, 170-400 ^ long, and 20-30^1 thick near the central . 

 tyle. Among the tri- to pentactines, forms with two fully developed and one 

 or two partly reduced rays are the most frequent. These spicules are 90- 

 260 fi in maximum diameter, very rarely as much as 350 n, on an average 206.7 n, 

 and their rays are 14-35 ju thick at the base, rarely up to 45 yu- 



The average measurements of the tri- to pentactines of the sponge-body 

 (223.6 n) and of the Palythoa (206.7 ju), given above, show that the former have 

 on the whole a larger maximum diameter than the latter. Also the rhabd-forms 

 show this, the average length of those of the sponge being considerably greater 

 than of those of the Palythoa. Apart from this it is to be noted that the Paly- 

 thoa spicules have stouter rays, and are more spiny than those of the sponge. 

 These facts seem to indicate : — 1 , that the more slender and less spiny acantho- 

 phores are young forms of the stouter and more spiny ones; 2, that none, or 

 only a few, of these young spicules, but many of the old spicules, are shed by the 

 sponge; and 3, that of the old, stouter, and more spiny spicules which are shed 

 and thus placed at the disposal of the Palythoa, the latter selects the smaller 

 (shorter) ones for building up its skeleton. 



