326 ANIMALIA RADIATA. 
form elevation. In a word, notwithstanding some irregulari- 
ties, and some very few exceptions—those of the Planaria 
and most of the Infusoria—traces of the radiating form are 
always to be found, which are strongly marked in the greater 
number, and particularly in Asterias, Echinus, the Acalepha, 
and the innumerable host of the Polypi. 
The nervous system is never very evident, and when traces 
of it have been apparently visible, it was also arranged in 
radii; most frequently, however, there is no appearance of it 
whatever. 
There is never any true circulating system. The Holo- 
thuria are provided witha double vascular apparatus, one 
portion of it being attached to the intestines and correspond- 
ing to the organs of respiration, and the other merely serving 
to inflate the organs which supply the want of feet. The 
latter is only distinctly visible in Ursinus and Asterias. 
Through the gelatinous substance of the Medusz we can see 
more or less complicated canals arising from the intestinal 
cavity; all this precludes the possibility of a general circula- 
tion, and in the great number of Zoophytes it is easily proved 
that there are no vessels whatever. 
In some genera, such as Holothuria, Ursinus, and in several 
of the Entozoa, we observe a mouth and anus, with a distinct 
intestinal canal. Others have an intestinal sac, but witha 
single opening serving both for a mouth and anus. In the 
greater number there is merely a cavity excavated in the 
substance of the body which sometimes opens by several 
suckers ; and finally there are some in which there is no 
mouth visible, and which can only be nourished by porous 
absorption. 
The sexes of several of the Entozoa or Intestinal Worms 
can be distinguished. ‘The greater number of the other Ra- 
diata are hermaphroditical and oviparous; some have no ge- 
nital organs, and are reproduced by buds or division. 
The compound animals, of which we have already seen 
some examples in the last of the Mollusca, are greatly multi- 
plied in certain orders of the Radiata, and their aggregation 
