REPORT ON THE TETRACTINELLIDA. 
Ixi 
wliicli has ceased to secrete its sclere ; variable numbers of actines proceed from the 
centrum, usually from four to twelve ; when, as is usual, only four or five are present, 
they proceed from one face of the centrum, viz., that which is directed centripetally, 
and terminate by abutting with expanded ends on the centrifugal face of the centrum 
of the desmas situated in the next row further inwards. From the centrifugal face of 
the centrum actines are seldom produced, but a thick bushy growth of spines replaces 
them, these more or less conceal the syzygial ends of the actines, rendering the nature 
of the zygosis obscure. 
Class IL Miceoscleees (Microsclera). 
Comparatively small or “ flesh ” spicules. 
These, which are perhaps the most interesting group of spicules, since it is from them 
that the megascleres have been derived, and since they still present us with a remarkable 
Fig. XII. — a, Globule ; 6, c, sigmaspires ; d, sigma of a Holothuroid ; e, centractinate sigma of a Holothuroid ; /, microtriod 
with curved rays, produced from e by overdevelopment of the centractine ; g, centractinate sigma of a sponge ; h, 
microtriod of Pfaa'na, retaining traces of its sigmate origin; i, microtriod ; y, orthodragma ; A, monolophous micro- 
calthrops ; Z, dilophous microcalthrops ; m, trilophous microcalthrops ; n, tetralophous microcalthrops ; o, candel- 
abrum ; p, toxa ; q, spirula ; r, microstrongyle, derived from a sigma ; s, spiraster ; t, amphiaster ; u, metaster ; 
«, 10 , plesiasters ; x, microcalthrops ; y, chiaster ; z, reduced chiaster ; a, saniclaster ; /3, anthaster ; y, mioroxea 
produced by reduction of /3 ; 6, oxyaster ; tJj micro’xea resulting from its reduction ; 0, spheraster ; centrotylote 
microxea resulting from its reduction ; X, pycnaster ; /u, microstrongyle resulting from its reduction ; tt, spherical 
sterraster, showing the nucleus situated in the hilum ; p. elongate sterraster. 
series of transitions within their own limits, are divided into two chief series, the radiate 
or astral, and the curvilinear or spiral. There are some few forms that cannot properly 
be said to belong to either of these groups, but they are so few and exceptional that it 
seems preferable to include them with the forms to which they are most nearly allied by 
descent, than to create special groups for their reception. 
