224 
and so on; but the reason, why there are only two plates next 
to the single primary plate, needs by no means be this, that it is 
a recapitulation of a phylogenetic stage with biseriate interambulacra 
(_ according to the “law of localized stages in ontogeny,” which 
Jack s on lays rather much stress on —) it may simply be due 
to the faet that there is no room for more plates liere. 
Jackson’s suggestion that the pluseriate interambulacium lepiesents 
a more specialized type than the biseriate finds, in faet, no suppoit 
in the forms actually known, and neither are there morphological 
reasons, which make that suggestion necessary. On the contrary it 
is more in accordance with general morphology to regard the many 
plates, more or less regularly arranged, (— the serial arrange¬ 
ment is perhaps not always so distinet as would appear from 
the connecting lines which Jack son adds in his analytical figures; 
in the Echinocystoida the arrangement is apparently not at all 
serial —) as the primitive condition from which the biserial anan- 
gement developed. In the family Lepidesthidæ a similar develop- 
ment is actually seen; in the most specialized of them all, Meek- 
echinus elegans, in which the ambulacra consist of no less than twenty 
columns, the interambulacra are only triseriate. A parallel develop- 
ment may be seen in the Cystidea, and even in Vertebrates a 
parallel can be pointed out, viz. in the morphology of the paired 
limbs. — On the other hånd, a pluseriate arrangement may well 
be the result of a special development from the biserial type. 
Tetracidaris is an example thereof. (From Vertebrate morphology 
a somewhat similar case is known in the many-jointed fingeis of 
Cetaceans). 
In a letter to me Jackson points out that theie is no em- 
bryological evidence for the pluseriate condition being the moie 
primitive; it might be suggested that, if it be really so, one might 
expect to find traces of a pluseriate condition in the young de\eloping 
Echinoid. I agree that such a most interesting stage, whereby the 
question would be definitely settled, has not been found. But it 
may also be remembered that scarcely any Echinoid as yet has 
