Ivi 
THE VOYAGE OF H.M.S. CHALLENGER. 
The different parts of the trisene (Fig. XL) are distinguished as follows : — The 
“rhabdome” (shaft) which corresponds to nearly the whole of the primitive rhabdus, i.e., 
the whole of the esactine and the acladose portion of the ecactine, and the “ cladome ” 
which consists of the three cladi (rays, teeth, arms). The point at which the axial rods 
of the cladi originate from that of the rhabdome is the “ genesis cladi ” or cladal origin. 
The cladi may themselves subdivide once, twice, or oftener ; usually dichotomously in 
one plane. The proximal or undivided part of a cladus is then distinguished as the 
protocladus, the distal division, in the case of a dichotomous cladus as the deuterocladus. 
Fig. XI. — A. Dichotrisene ; r.o., origin of the rhabdome; c.o, origin of the cladome ; Cl., the cladome (this is included by 
the dotted lines, the rest of the spicule is the rhabdome) ; pr., protocladus ; d(r., deuterocladus. £. Anatrisene ; c.o., 
cladal origin ; ch.-ch., chord; cA.-c.o., length of cladus; dotted line from s. to the chord, sagitta. C. Protriajne ; 
C.O., cladal origin; c.o.-s., sagitta ; ch.-ch., chord. 
in a twice dichotomous cladus as the tritocladus, and in a thrice dichotomous cladus, the 
tetracladus. 
An imaginary straight line joining the ends of two of the cladi is termed the chord. 
A perpendicular from the origin of the cladome to the chord is the sagitta. 
A straight line drawn from the cladal origin to the end of a cladus is taken as its 
length. 
The length of the cladus, of the chord, and of the sagitta are the chief elements in the 
measurement of the cladome. 
