ECHINODBRM MORPHOLOGY. 
329 
Crinoids, one, namely, which has not been thought to exist 
since the researches of Agassiz and Loven. While the 
Crinoid calyx increases by the additions of plates on the 
aboral side of the oral plates that of an Urchin grows very 
differently : 
Hier entfernen sich die Oralplatten der Crinoideen homologen Genital- 
platteii immer weiter von dem Munde, indem die Bildung des Perisoras des 
erwacbsenen Tbieres, genauer die Bildung der Interambulacralfelder, an der 
adoralen seite der Genitalplatten erfolgt.’’ 
This must not be understood to mean (as it well might 
do) that the genital plates have the same primitive relation 
to the mouth of an Urchin as the orals have to the mouth of 
a Crinoid and only subsequently become separated from it, 
for this is far from being the case. 
The old view that the genitals of an Urchin represent the 
Crinoid basals involves none of the difficulties inseparable 
from Ludwig’s new one. In both Crinoids and Urchins^ 
new plates are added on the adoral side of the interradial 
ahactinal plates, viz. the genitals of the latter and the basals 
of the former, which Ludwig, however, considers to be unre- 
presented in the Urchins. 
The only other out of the Reihe wichtiger Folgerungeii 
fiir die vergleichend-anatomische Auffassung der Skeletre- 
gionen der verschiedenen Echinodermgruppen” that Ludwig 
deduces from the homologies which he asserts is the follow- 
ing^ : — Dass das perianale P’eld der Echiniden dem gesamm- 
ten Perisom der Ophiurensheibe mit Ausnahme der Arme 
und der Mundschilder homolog ist.” 
There seems to me to be something wrong here. Perhaps 
it is only that Ludwig and I understand the expression 
perianale Feld” in different ways. I take it to mean the 
ring of genital and ocular plates, and, as Ludwig considers 
the former to be homologous with the mouth-shields of the 
Ophiurids, I am somewhat puzzled as to the meaning of the 
above passage. 
The above are many inconsistencies which become appa- 
rent upon a critical examination of this new theory of 
Echinoderm morphology. It is difficult to understand how 
Ludwig can insist on the homology of the oral surfaces 
of Urchins, Starfishes, and Ophiurids, and yet insist even 
more strongly that plates around the mouth of the latter 
are homologous with plates around the anus on the aboral 
^ This Journal, New Series, vol. xix, pp. 2G — 28. 
’ ‘ Zoologiseber Anzeiger,’ 1879, ii, p. 542. 
