194 FUNCTIONS AND INSTINCTS. 



zoophytes ; but when we recollect that they 

 cannot, like the infusories and polypes, be pro- 

 pagated by cuttings and offsetts, this seems to 

 indicate an animal substance in which the nervous 

 molecules are less dispersed, and that some ten- 

 dency to nervous centres has been established. 

 In the upper classes of invertebrated animals, 

 indeed, many will reproduce an organ when 

 mutilated, and some even a head, but none but 

 the polypes and infusories multiply themselves 

 in the way above stated. It seems, therefore, 

 most advisable to adhere to Lamarck's system, 

 by considering the animals in question, as form- 

 ing a group by themselves, and to adopt his 

 name of Radiaries. 



These are distinguished from the class imme- 

 diately preceding, the polypes, by being limited 

 as to their growth to a certain standard, as to 

 their form by the general appearance of radiation 

 they usually present, being either divided into 

 rays, as in the star-fish ; or having rays exhibited 

 by their crust as in the sea-urchins ; or embedded 

 in their substance, forming appendages to their 

 viscera, as in the sea-nettle or jelly-fish. They 

 have not, like the polypes, a terminal mouth or 

 orifice surrounded by food-collecting tentacles ; 

 but one placed, most commonly, underneath 

 their body. Their digestive organs are distinct 

 and more complex. They are never fixed, and 

 are to be met with only in the sea and its estu- 



