454 CHARACTERS OF INVERTEBRATE ANIMALS 



it would seem, with the marked development of a continuous 

 exoskeleton, between the parts of which there are not, as in 

 Arthropods, well-marked joints permitting of free movement. 



The nervous system is of very primitive type, and arranged, 

 like the other internal organs, in accordance with the radial 

 symmetry of the animal. It is intimately connected with the 

 skin, of which, in fact, it may simply be regarded as a thicken- 

 ing, and its best-marked portions consist of a ring round the 

 mouth, and five radial nerves, one of which runs along each arm. 

 But the entire body is invested in a nervous sheath, consisting 

 of a delicate net-work lying in the deeper part of the skin. The 

 chief sense organs are the tube-feet, which appear to be, among 

 other things, tactile structures — indeed there is an odd tube-foot 

 at the end of each arm which may definitely be called a tentacle, — 

 and the eye-spots already noted. 



What are known as "comet" forms of Star- Fish are not 

 infrequently met with, i.e. specimens in which from one to four 

 arms are smaller than the remainder. The small arms are new 

 ones growing in place of others which have accidentally been 

 lost. Lower animals often possess considerable powers of re- 

 generating mutilated parts, and this is to a limited degree the 

 case even among some of the Vertebrates. 



Living Echinoderms are arranged in five classes: — i. Star- 

 Fishes (Asteroidea) ; 2. Brittle-Stars (Ophiuroidea); 3. Sea-Urchins 

 (Echinoidea); 4. Sea-Lilies and Feather-Stars (Crinoidea); Sea- 

 Cucumbers (Holothuroidea). 



Class I.— STAR-FISHES (Asteroidea) 



These conform in character to the described type in many 

 at least of the particulars. They are found in all seas, and 

 the majority are inhabitants of shallow water, though some, 

 including the largest species, are found at considerable depths. 

 There are generally five arms, but this is by no means a 

 universal rule. A common British species, for instance, the Sun- 

 Star iySolaster pappostis), possesses no less than thirteen. The 

 name has been given on account of its resemblance to the con- 

 ventional symbol by which the sun is usually represented, i.e. a 

 circle with a number of radiating lines. 



