262 
Holothnria floridana 
of the table (Fig. 42) but with the central rod shorter and more slender and the 
prongs shorter, wider and thicker. The diverging branches of the prongs unite 
to form holes and thus is developed the 4-holed symmetrical Type a (Fig. 37) in 
which the 2 central holes are about twice the size of the 2 terminal holes. Of 
this type there are 2 dorsally and 6 °/„ ventrally. Plates of this stage in 
general have not the perfectly smooth contour of Fig. 37 but show one or more 
bifurcating branches which will unite to make 2 additional iioles at each end and 
thus produce the 8-holed symmetrical Type h (Fig. 38). Eighteen per cent, of 
both the dorsal and vential developmental stages are of Type b. The majority of 
Type b plates have the peripheral prongs indicating growth toward the complete 
plate, and also bars running into the larger central holes, as well as occasionally 
into the smaller holes (Fig. 38a). 
Type a* with 4 holes asymmetrically distributed does not occur in the dorsal 
series but does in 6 "/o of the ventral, and Type b* with 8 holes asymmetrically 
distributed is found in 5°/„ of the dorsal and 15 7o of the ventral. Type c includes 
all of the plates with 5 — 8 holes indefinitely distributed, part of them showing the 
regular contour which might be taken to indicate a fully developed plate, and part 
of them with the prongs of growth of an incomplete plate. Type c constitutes 
76% of the dorsal and 00°/^ of the ventral plates. It is quite possible that many 
of these types, especially those with regular contours and of large size, are developed 
plates (" buttons "), but since Types a and b are clearly two well-marked develop- 
mental stages, I have included all plates with 8 or less holes in the same general 
group of perforated plates in the process of development. Their mean number of 
holes is about 6'5 both dorsally and ventrally, their mean dimensions 29/i x 23/4 
dorsally and 27/i x 2l/x ventrally with practically no difference in the standard 
deviations in the two regions. 
2. Structure and Variation of the Developed Perforated Plates. 
The plate becomes developed by the addition of more holes to Type b, while, 
with increasing age all of the holes may be gradually filled in with lime (Figs. 39, 
40, 41). The mean number of holes is 13 dorsally and 15 ventrally and the mean 
dimensions 29 fi x 24/i. dorsally and 27 x 22 fi ventrally, with the standard devia- 
tions somewhat greater in the ventral region. 
20°/^ of the dorsal and 10°/^ of the ventral plates have the peripheral projec- 
tions which indicate that additional holes may be in process of formation. Such 
a plate, much larger than the average, is shown in Fig. 34. In many cases a bar, 
or sometimes 2 or 3 may grow out into each of the 2 primary central holes 
(Fig. 34). The bar may grow through the hole, fusing with the opposite wall, 
and thus forming 2 holes in place of the normal single hole. 
In the majority of these developed plates it is easy to trace the fundamental 
Type b (cf. PI. VIII. Fig. 7, d, e of H. africaim Theel, 1886). 
Sometimes the initial forked bar is much thicker than usual (Fig. 35) and 
after the fusion of the terminal branches the 2 primary holes formed are very 
