18 



THE SPONGES. 



difference cannot be regarded as of importance, since the character of 

 the pinule is the same in both sponges. 



The larger variety of macramphidisc has a total length of 200 to 320 /x. 

 The length of the bell-shaped umbel is slightly less than ^ the total 

 length of the spicule, and 1 t 6 q times the width of the umbel. The shaft 

 bears at its middle a circle of small protuberances, and usually other 

 protuberances are scattered irregularly along it. In some cases there are 

 three circles of protuberances, one in the middle, and one toward each 

 end of the shaft. The shaft is very slender, excluding tuberosities about 

 5 fi thick. The spicule is abundant in the dermal membrane, and is present 

 also in the parenchyma, particularly in the peripheral region. 



The ellipsoidal form of macramphidisc has a total length of 60 to 

 80 fji. The greatest width of the umbel is about equal to its length, and 

 the umbels nearly meet at the equator of the spicule. The arrangement 

 of the protuberances on the shaft varies. More commonly they are aggre- 

 gated into one group at the middle. Frequently, however, there may be 

 two such groups, on opposite sides of the middle point of the shaft. In 

 some spicules the protuberances extend nearly over the entire length of 

 the shaft. The spicule is abundant in the dermal membrane, and also 

 present in the parenchyma. 



Small amphidiscs, which collectively may be referred to as micram- 

 phidiscs, are abundant in the parenchyma. How abundant they are in 

 the dermal membrane is impossible to determine, since the surface is 

 injured. The smallest of these spicules are micramphidiscs of the common 

 type, with umbels about -J- the total length of the spicule, or somewhat 

 less. The total length of the spicule is 16 to 24 /x. The umbels are 

 as wide as deep, and with numerous (about 16) teeth. The shaft is gen- 

 erally smooth and enlarged in the middle ; in some of the larger spicules 



spinose in the middle. 



Other small amphidiscs in which the umbels 



closely approach or reach the equator of the spicule, and in which the 

 shaft is spinose, form a series leading up from a length of 30 /x to the ellip- 

 soidal form of macramphidisc 60-80 /x long. As an example may be given 

 a spicule measuring 30 /x long by 18 /x wide, in which the teeth of the 



opposite umbels meet at the equator. 



Still another type of small amphi- 



disc 30 to 50 /x long, resembling in general the elongated form of macram- 

 phidisc, is abundant in the parenchyma. The shaft bears sharp tuberosities, 

 and the umbels are bell-shaped, somewhat deeper than wide, and some- 









