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2. No articulated limbs or visceral clefts. 
3. That in development the embryo does not present a 
longitudinal groove. 
4. That the anterior part of the alimentary canal is sur- 
rounded by the central part of the nervous system. 
5. That the body has a calcareous protection, and not a hard 
chitinous envelope. 
6. That it does not consist of a longitudinal series of similar 
segments, either internally or externally. 
The question to what other animals the echinoderms are 
most nearly related, or whether they are so distinct as to merit 
the attribution to them of a distinct primary division of the 
animal kingdom, is a difficult and vexed question. 
All things considered, it seems probable that their nearest 
allies are to be found in certain of the lower worms, such as 
those amongst which the tapeworm is classed, and which 
together form the compound and somewhat unsatisfactory 
group the Scolecida. Only in these, besides the echinoderms, 
is found that complex and singular mode of reproduction 
which consists of the development of a primary larva as a 
suitable nidus for the development of the future adult form. 
Moreover, many of these Scolecida present us with a double 
set of vessels, which appear to answer to the ambulacral and . 
pseudhaemal systems of the echinus ; there being in many 
one set not contractile, but ciliated internally (answering to 
the ambulacral vessels), and another set which are con- 
tractile, but not ciliated internally (answering to the pseud- 
hjemal vessels). On the whole, then, if the echinoderms are 
associated with any other animals, it will be with the Scole- 
cida. This has been done by Professor Huxley, who has pro- 
posed the name Annuloida , to designate the whole formed by 
the union of the echinoderms and Scolecida in one great group. 
As to the subdivisions of the class Echinodermata , the existing 
forms are easily separable into five groups or orders. The first 
of these is termed Echinulea , and comprises all those echino- 
derms which in their organisation most closely resemble our 
type — the echinus. The common characters of this order are — 
that the body is spheroidal, or at the least discoidal ; that the 
integument has so many calcareous plates as to form a shell ; 
that there is invariably an anus ; that the primitive larva has 
the form of a pluteus , as just described ; and, finally, that the 
ambulacra extend over the greater part of the shell, or, in more 
technical language, that the ambulacral region is in excess of 
the antambulacral region. 
The second order (fig. 6) is the Holothuridea , which may be 
described as echini with the integument softened and nearly 
