REPORT ON THE TETRACTINELLIDA. 
liii 
Group 2. Teteaxons (Tetraxona). 
Megascleres in whicli growth is directed from an origin in one direction only, along 
four axes arranged as normals to the faces of a regular tetrahedron. 
Forms which are geometrically triaxons, i.e., produced by growth from an origin in 
one direction along three axes lying in one plane, are regarded as reduced tetraxons. 
Group 3. Teiaxons (Triaxona). 
Megascleres in which growth is directed from an origin in both directions along three 
rectangular axes. 
The growth may take place in both directions along each axis, or in one direction 
only, or along one or more of the axes in one direction and along the remainder in both 
directions. One or more axes may be suppressed. 
Group 4. Polyaxons (Polyaxona). 
Megascleres in which radiate growth from a centre proceeds in several directions. 
Group 5. Spheres (Sphserae). 
Megascleres in which growth is concentric about the origin. 
Group 1. Classification of The Monaxons. 
I. Diactine {diactina ). — A monaxon in which growth proceeds in both directions 
along the axis. For this the term rhabdus is substituted as a convenient abbreviation. 
II. Monactine [monactina ). — A monaxon in which growth proceeds in one direction 
only along the axis. For this form the term style {stylus) proposed by Vosmaer is 
substituted. 
Modijications of the Rhahdus. 
When the rhabdus is arranged within the sponge so that one end is directed centri- 
fugally and the other centripetally, the centrifugal actine is distinguished as the ecactine 
and the centripetal as the esactine. Both actines may be of similar shape and size, the 
rhabdus is then isoactinate, or one may be larger than the other, and the rhabdus is 
anisoactinate. It is seldom possible to discover the position of the origin of the spicule 
exactly, but the distinction between iso- and aniso-actinate forms is not made unless the 
difference is obvious. 
