It ADI AT A. 393 



DIVISION IV. RADIATA. 



THIS division of the Animal Kingdom comprehends a great 

 number of beings, of organization more simple than the prece- 

 ding classes. However different otherwise in their structure or 

 form, they seem (according to Cuvier) to correspond in the 

 character of having all their parts disposed around a common 

 axis in two or more rays, or in two or more lines extending 

 from one extremity to the other. Even the intestinal worms 

 have at least two tendinous lines or nervous threads arising from 

 a circle round the mouth ; many have four suckers around a 

 prominence in the form of a proboscis ; and notwithstanding 

 some irregularities, there is always found in the animals arrang- 

 ed under this division traces of a radiated form, indistinctly 

 marked in some, but in others, such as the Asterioc^ the Echi- 

 ni, and the Polypi^ strikingly perceptible. 



The nervous system in the animals of this division is never 

 very evident ; and of a circulation by vessels, as in the previous 

 classes, there is no trace. The Holothurice have two vascular 

 appendages, one attached to the intestines, and corresponding 

 to the organs of respiration, and the other serving for the infla- 

 tion of organs analogous to feet. The last of these only appears 

 distinctly in the Echini and the Astericc. In the gelatinous 

 substance of the Medusae are seen tubes more or less complicated, 

 connected with the intestinal canal ; but none of these appear- 

 ances are conceived to have any strong analogy with the circu- 

 lating vessels of the higher animals. 



Some genera, such as Holothuria, Echinus, and many intes- 

 tinal worms, have a mouth and anus, with a distinct intestinal 

 canal ; others have an internal pouch, with only one opening, 

 serving the purposes of a mouth and anus ; but in the greater 

 number there is only to be discovered a hollow cavity in the 

 substance of the body, opening sometimes by many suckers or 

 pores. Finally, in the lowest races of the animal kingdom, even 

 this simple organization disappears, and nutrition seems to be 

 accomplished by absorption, in the manner of vegetables. 



In regard to their reproduction, sexes have been observed in 

 many of the intestinal worms ; others are hermaphrodite and 



