RADIATA. 463 



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 Holothuria are 

 provided with a double vascular apparatus, one portion of it being 

 attached to the intestines and corresponding to the organs of respira- 

 tion, 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 Medusae we can 

 see more or less complicated canals arising from the intestinal cavity ; 

 all this precludes the possibility of a general circulation, 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 with a 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 Radiata are herma- 

 phroditical and oviparous ; some have no genital organs, and are repro- 

 duced by buds or division. 



The compound animals, of which we have already seen some examples 

 in the last of the Mollusca, are greatly multiplied in certain orders of 

 the Radiata, and their aggregation produces trunks and expansions 

 forming all sorts of figures. It is to this circumstance, together with 

 the simple nature of the organisation in most of the species, and the 

 radiating disposition of their organs which reminds us of the petals of 

 flowers, that they owe their name of Zoophytes^ or Animal-plants, by 

 which we merely mean to express this apparent affinity, for as Zoo- 

 phytes enjoy the sense of touch and the power of voluntary motion, 

 mostly feed on matters which they have swallowed or sucked, and 

 digest them in an internal cavity, they are certainly animals in every 

 point of view. 



The greater or less degree of complication in Zoophytes has occa- 

 sioned their division into classes ; but as all the parts of their organisa- 

 tion are not yet well known, those sections cannot be characterised 

 with as much precision as those of the preceding divisions. 



