REPORT ON THE TETRACTINELLIDA. 
59 
same plane as those at the other ; but the one pair are trichotomate and the other 
simple. The other presents a rhabdome O'OllS mm. long, bearing two cladi as long as 
itself at one end, but terminating in a sharp point without branching at the other. The 
cladi each bear two very minute spines, one on each side, near the pointed extremity ; 
in other words, they are trichotomous. 
Both these forms suggest an origin from some microsclere similar in general 
character to the large asters of Thenea, or Plakina. Amongst the transitional forms 
from a spiraster to an oxyaster in Tlienea, one frequently meets with examples in 
which a slender rhabdome bears two cladi at each end ; if the rhabdome instead of 
shortening and aborting as in Thenea should increase in size and an additional cladus 
appear at each end, a simple form of amphitriaene would result. On the other hand, the 
occurrence of irregular amphitriaenes in Tetilla stipitata suggests another origin for these 
forms, and they may have been derived from a triaene. The curved form of the cladi 
observed in the immature triradiate form of amphitriaene mentioned above, suggests the 
possibility of a third mode of origin. Let a sigmaspire pass into a sigma, and from analogy 
with the calcareous sigmata of Echinodermata and Nudibranchiate Molluscs, we may 
expect this in some cases to develop an actine from the middle of its convex side. This 
would give a triradiate form, with two rays curved. Let these develop spines, one on 
each side of their termination, and the immature form of amphitriaene described results. 
Thus, there is considerable room for speculation on the mode of origin of these spicules, 
but without a basis of embryological data speculation is likely to prove barren. 
Suborder II. ASTEOPHOEA. 
Choristida in which one of the microscleres is some form of aster. 
Demus I. Steeptastrosa. 
Astrophora in which one of the microscleres is some form of spiraster. 
Family I. Theneid^, 
The ectosome never forms a cortex; the mesoderm is a collenchyma; the flagellated 
chambers eurypylous. 
Thenea, Gray. 
Theneidse of symmetrical form, with one or more distinct oscules,and with specialised 
pore-areas, in addition to pores generally dispersed. The distinctive spicules are 
dichotrisenes, which are arranged together with the other megascleres in radiating fibres. 
