294 LIFE BY THE SEASHORE. 



One interesting point is that, as the description shows, 

 the branchial sac has a double function, being both respira- 

 tory and nutritive, and that through it there constantly 

 flows a food- and oxygen-bearing current. This fact is 

 known and appreciated by a little water-flea, or Copepod 

 (Notodelphys ascidicola), which takes up its abode within 

 the branchial sac of various sea-squirts. It is hardly a 

 parasite, for it does not seem to injure the sea-squirt, but 

 seeks and obtains shelter, as well as abundant oxygen and 

 food from the incoming current. The habit might be de- 

 scribed as a first essay towards the adoption of the parasitic 

 mode of life, for it is probable enough that many parasites 

 began by merely seeking shelter. Such a method of life is 

 not peculiar to Copepods, for there are also fish which 

 similarly seek shelter within the cavities of sea-anemones 

 and sea-cucumbers, and the interesting case of the crab 

 and the mussel has been already noticed (p. 202). In all 

 such cases the cavity used for shelter must be one in which 

 there is an abundant supply of sea-water periodically re- 

 newed, by means of which the messmate can both breathe 

 and feed. 



A considerable number of simple sea-squirts as opposed 

 to those forms which produce colonies are to be found on 

 the shore rocks, so that we can pick out one or two only. 

 Perhaps the commonest in most places is the " gooseberry " 

 sea-squirt (Styelopsis grossularia), to be found on rock 

 surfaces as a little bright red body, often so covered with 

 mud that nothing but the two bright red orifices is to be 

 seen. When touched these disappear in the surrounding 

 mud after squirting out a sudden jet of water. With a 

 little care it is possible to remove a few specimens without 

 injury, and make out something of the internal anatomy. 

 It will be found that the body is nearly spherical, and the 

 two apertures are placed close together, and are both four- 

 lobed. If with a pair of scissors you clip the animal in 

 two, you will see that the branchial sac has one deep fold 

 in it, as well as some other indistinct ones, and that the 

 inner surface of the body-wall has, on its surface, little 

 scattered masses ("polycarps"), which are the reproductive 

 organs, and are confined to the right side of the body-wall. 

 These points are worthy of notice, because they serve as 



