PHYLUM CCELENTERATA 167 



derm). Other cells pour out secretions which soften and 

 liquefy the food. If there are parts which cannot be 

 digested they must be thrown out through the mouth for 

 there is no other opening into the enteron, except the canal 

 which leads down the stalk to connect with other indi- 

 viduals. The digested food is absorbed by the entoderm 

 and part of it is there assimilated, but some soaks on 

 through to nourish the ectoderm on the outside of the 

 body. The layers of Obelia's body wall are so thin that 

 there is no need for any separate circulatory or excretory 

 systems. 



Self-protection. An Obelia colony is invested over its 

 entire surface by the peritheca, a tough horny secretion 

 which forms a branched system of tubes and protects the 

 soft tissues within. This exoskeleton has little openings 

 at the tips of the branches so that the hydranths and 

 gonangia may communicate with the exterior. Each hy- 

 dranth is enclosed by a little cup, or hydrotheca (Fig. 67, ti) 

 into which it may withdraw when threatened with danger. 

 The peritheca also keeps the delicate body from drying out, 

 if it is by chance left exposed at low tide. If. this protective 

 covering were too rigid, it would be a burden and is there- 

 fore jointed (j) near the base of each branch so that 

 movement is permitted. 



The nematocysts (Fig. 69) help to protect Obelia from 

 small creatures which might otherwise prey upon it,. but 

 some animals appear to be immune to their feeble stings. 

 Certain crabs use Obelia and other hydroids as a favorite 

 food. Some snails which have no shells not only devour the 

 hydroids, but do so without discharging the nematocysts; 

 and some species are even able to pass them out to their 

 own backs where they use them " second-hand " to sting 

 their enemies. 



The attachment of Obelia to some solid object protects 

 it from certain dangers. It cannot be swept up on the 

 beach by storms and is never carried out to sea, but it 

 thus sacrifices its ability to search about for food. 



