HEDGEHOG-SKINNED ANIMALS 



355 



effected, and which is arranged as shown in fig. 873. Later on, 

 the body grows out into a number of narrow "arms", on to 

 which the ciliated band is continued. These are directed for- 

 wards, and supported by an internal skeleton of calcareous rods, 

 which are united together -in the hinder part of the body. The 

 larva is known as a Pluteus. After a time the body of the adult 

 star-fish begins to appear as a thickening in the body-wall of the 

 larva, and as this increases in size the ciliated arms, with their 

 supporting skeleton, are gradually absorbed. The internal organs 

 of the Pluteus become the corresponding parts of the adult Brittle- 

 Star. In some creatures of this kind there is not a free-swimming 

 larva, but the fertilized eggs 

 gradually assume the adult form, 

 being sheltered in the meantime 

 in pouches situated between the 

 bases of the arms, and already 

 spoken of in regard to breath- 

 ing (see vol. ii, p. 414). This 

 kind of life-history is, however, 

 exceptional among Brittle- Stars, 

 and has resulted from abandoning 

 the method of indirect develop- 

 ment, i.e. through a larval form, 

 for direct development, by which 

 the adult stage is gradually at- 

 tained without any startling trans- 

 formations. The use of free-swimming larvae is doubtless to help 

 in spreading the species, but the mortality of such immature forms 

 is exceedingly large. The more sheltered direct type of develop- 

 ment presents such advantages in this direction that we can un- 

 derstand why, in some species, it has superseded the larval type 

 altogether, as, for example, in our commonest shore form (Am- 

 phiura squamata). If the eggs of this particular species developed 

 into Plutei the chances of one of these becoming adult and reach- 

 ing a suitable spot in which to live would be very remote. Direct 

 development is here distinctly more favourable to survival, and 

 as this species may also be said to possess a dwelling, since it 

 is commonly found under stones near low-water mark, the chance 

 of a given egg becoming an adult is still further increased. 



Sea- Urchins (Echinoidea) usually have much the same sort of 



Fig. 874. Young Sea- Urchins (Hemiaster caver- 

 nos-ns] sheltered in Grooves between the Spines of their 

 Mother 



