246 THE STUDY OF ANIMAL LIFE CHAP, xn 



Another characteristic of the Echinoderms is the 

 strangeness of the larval forms. For not only are the 

 larvae very different from the parents, and very remark- 

 able in form, but they do not grow directly into the 

 adult. The development is ' indirect," the larva does 

 not become the adult ; the foundations of the adult are 

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

 or partly rejected. 



In a relatively small number of cases, among star- 

 fishes, sea-urchins, and sea-cucumbers, there is some 

 measure of parental care, the young ones remaining for 

 a prolonged period externally associated with the mother. 

 In these cases the usual free -swimming larval stages are 

 suppressed and there is often at least some obvious reason 

 for this. Thus the parental association is seen in a 

 number of Arctic and Antarctic forms where the abun- 

 dance of ice is prejudicial to the success of delicate larval 

 forms. 



Not only the starfishes but also the brittle-stars and 

 the feather-stars often surrender their arms when cap- 

 tured, or even when slightly irritated, and a part or a rem- 

 nant can in favourable conditions regrow the whole. 

 The Holothurian Synapta breaks readily into pieces, and 

 others contract themselves so forciblv that the internal 



/ 



organs are extruded. 



The relations of Echinoderms to other animals are 

 many. A little fish, Fierasfer, goes in and out of Holo- 

 thurians ; the degenerate Myzostomata form galls on 

 the arms of Crinoids ; starfishes 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. 



0. Arthropods. Lobsters, centipedes, insects, spiders, 

 ;i ni- (( - with the Annelid ' worms " in being built up of a 

 series of rings or segments. Some or all of these seg- 

 ments bear limbs, and these limbs are jointed, as the 

 term Arthropod implies. The skin forrns an external 



