188 MARINE BIOLOGY OF THE SUDANESE RED SEA. 
until it lies radially, along the side of a flagellated chamber, and at right 
angles to the facial plane. The oral rays, on the other hand, still lie in the 
cortex, and, at any rate so far as their distal portions are concerned, in 
the primitive facial plane. The proximal part of the oral rays is frequently 
gradually curved between the tangential and radial positions. The oral angle 
of the prochiact is of course the angle in the primitive facial plane between 
the paired rays. 
a. b. C. 
Fig. 1.—Diagram of the evolution of secondary sagittal triradiate (c) through a 
prochiact (2) from an ordinary sagittal triradiate (a). 
A further development of this spicule, however, to form what we may term 
a secondary ™ sagittal triradiate, takes place by the gradual widening of the 
primitive oral angle until it becomes 180°, i. e. until the paired rays come to 
lie exactly opposite each other, and once more in one plane with the basal 
ray (fig. 1, c). In this way a new or secondary oral angle is developed, quite 
distinct from the primitive oral angle of the primitive facial plane, since it is 
bounded by different regions of the rays, the regions bounding the primitive: 
oral angle now forming the sides of the rays. There is also established a 
secondary facial plane at right angles to the original one, by this alignment of 
the paired rays. The spicule thus formed is in shape a typical sagittal tri- 
radiate, and I believe that such spicules form the characteristic subdermal 
sagittal triradiates of the family Heteropide. In practice it must often be 
impossible to distinguish between secondary and primary sagittals. 
A discussion of this view as to the nature and origin of the prochiacts, 
and also the evidence which’ has led to its adoption, will be given later on, 
when dealing with the evolution of the family Heteropide. 
* The term secondary is here used only in a general sense, and it must be understood that 
the secondary sagittal spicules referred to in this paper are not of necessity equivalent to- 
secondary sagittal spicules described by other authors. 
