51 
is a central plate or not in the Clypeastroids. Three species have 
been studied as regards their development, viz. Echinocyamus 
pusillus , by T h é e 1, Echinarachnius parma , by Fewkes, and 
Mellita testudinata, by C a s w e 11 G r a v e; thé latter author, however, 
does not give any information of the development of the apical system x ). 
In the young Echinocyamus the whole apical side is covered 
by a single large plate, in which the water-pore is situated. (PI. IX. 
Fig. 107 of Théel’s work „On the development of Echino¬ 
cyamus pusillus“). This plate originates in a most remarkable way 
from the dorsal rod of the larval skeleton (PI. VIII. Figs. 103 — 
104, p. 57). There is nothing in this remarkable development to 
prove that this is a real central plate, homologous to the central 
plate of Salenia. The development of the plates of the anal 
membrane is not described by Théel. I have examined some 
young specimens of 1,5—2 mm length as to this and find there a 
circle of nearly equal sized plates, among which a larger suranal 
plate cannot be pointed out. 
In Echinarachnius parma the matter would seem to be very 
different. Fewkes (Preliminary Observations on the Development 
of Ophiopholis and Echinarachnius * 2 ) ascertains (p. 141) the existence 
of a „central plate, „centrale", of pentagonal outline, around which 
are arranged a ring of five plates, closely fitting to the central 
plate“. The figure to which Fewkes refers (PI. VII. Fig. 16) 
seems, however, to show beyond doubt that his interpretation is 
wrong. What he designates as the central plate is certainly of 
pentagonal outline, but there is no trace of calcareous network 
seen therein, and since he otherwise describes and figures the calca¬ 
reous network very carefully there has evidently been nothing of 
the kind in this space. In the figure 11, PI. VIII, are shown „a 
central and five peripheral plates in the (its) apical region", which 
x ) Some points in the structure and development of Mellita testudinata. 
Johns Hopkins Univ. Gire. No. 157. (Vol. XXI). 1902. 
2 ) Buil. Mus. Comp. Zool. XII. 1886. 
4 * 
