238 The Study of Animal Life part iii 



adult are laid anew within the body of the larva, which is absorbed 

 or partly rejected. 



Not only the starfishes but also the brittle -stars and the 

 feather-stars often surrender their arms when captured, or even 

 when slightly irritated, and a part or a remnant can in favourable 

 conditions regrow the whole. The Holothurian Synapta breaks 

 readily into pieces, and others contract themselves so forcibly that 

 the internal organs are extruded. 



The relations of Echinoderms to other animals are many. A 

 little fish, Fierasfer, goes in and out of Holothurians ; the de- 

 generate Myzostomata form galls on the arms of Crinoids ; star- 

 fishes are deadly enemies of oysters. On the other hand, some 

 sea-snails and fishes prey upon Echinoderms in spite of their 

 grittiness. Except that the unlaid eggs of some sea-urchins are 

 edible, and that some sea-cucumbers are considered delicacies, the 

 Echinoderms hardly come into direct contact with human life. 



5. Arthropods. — Lobsters, centipedes, insects, spiders, agree 

 with the Annelid " worms " in being built up of a series of rings 

 or segments. Some or all of these segments bear limbs, and these 

 limbs ' are jointed, as the term Arthropod implies. The skin 

 forms an external sheath or cuticle of a stuff called chitin, and 

 this firm sheath helps us to understand how the limbs became 

 well-jointed. The chitin seems in some way antagonistic to the 

 occurrence of ciliated cells, for none seem to occur in this large 

 series unless it be in the strange type Peripatus. The chitin has 

 also to do with the moulting or cuticle-casting which is common 

 in the series, for the cuticle is generally rigid and does not expand 

 as the body grows, hence it has to be cast and a new one made. 

 Finally, Arthropods have a nervous system like that of Annelids — 

 a double dorsal brain connected by a ring round the gullet with a 

 double chain of ganglia along the ventral surface. But the life of 

 most Arthropods is more highly pitched than that of Annelids. 

 The sense-organs are more highly developed, brains are larger and 

 more complex, the ganglia of the ventral chain tend to become 

 concentrated ; there is division of labour among the appendages ; 

 there are new internal organs such as ». heart ; the whole body 

 is better knit together. A crayfish may part with his claw and 

 grow another in its place, but the animal will not survive being cut 

 in two as some kinds of Annelids do. 



The series includes at least five classes : — 



Crustacea, almost all aquatic, and breathing by gills. 



Protracheata, represented by the genus Peripatus. 



Myriapoda, centipedes and millipedes. 



Insecta, more or less aerial. 



Arachnida, spiders, scorpions, mites, etc. 



