RESPIRATION IN SHORE ANIMALS 201 



has no very precise significance, and may be applied to 

 practically any delicate extension or fold of the body surface 

 through which an exchange of gases might reasonably be 

 supposed to take place. Gills or respiratory outgrowths 

 may occupy almost any position on the body. They are 

 superficially similar in fishes and in the larger crustaceans, 

 not only in situation, but also in appearance, and further 

 also in the manner in which they are protected — a conver- 

 gence which, as we shall see later on, has had interesting 

 consequences in the two groups. In both Isopod and 

 Amphipod crustaceans, many of which are amphibious, 

 the gills, though not homologous in the two groups, are 

 placed beneath the body and have the form of thin plates. 

 These gill-plates seem almost equally well adapted to 

 breathing either moist air or water, since those of Ligia, for 

 instance, differ little from those of permanently aquatic 

 forms. It is interesting to notice, however, that in the 

 related terrestrial wood-louse, Porcellio scaber, the respiratory 

 plates are traversed by fine branching tubes, which suggest 

 the tracheae of insects. In molluscs, as Semper (1899) 

 points out, there are no less than five forms of gills, all 

 morphologically different. We may mention those of 

 bivalves, folded plates perforated in a complex manner and 

 situated within the shell (see p. 196) ; those of most marine 

 shelled Gasteropods and of Cephalopods which for the most 

 part resemble the typical form, and those of naked marine 

 molluscs (Doris, Molis) which are extensions of the dorsal 

 body surface. The parapodia of Polychaet worms are gene- 

 rally supposed to have some respiratory significance, especially 

 the cirri, which are sometimes greatly elongated as, for 

 example, in Cirratulus. Tubicolous Polychaets have typi- 

 cally a " crown " of tentacles, or " gills," which project 

 from the open end of the tube. The term " gill " expresses 

 the general opinion as to the function of these appendages, 

 but they evidently serve other purposes besides that of 

 respiration (tube-building, food- wafting). A certain amount 

 of respiration is no doubt effected in starfishes, sea-urchins, 

 and brittle-stars by means of the tube-feet ; starfishes have. 



