RESPIRATION 149 



stand conditions of anoxia for a week or more. The danger of desiccation 

 is more urgent than temporary anoxia in littoral species. 



Aquatic gills are delicate structures unsuitable for aerial respiration, 

 since they collapse in air with consequent reduction of surface area. Pro- 

 gressive adaptation towards terrestrial life is seen in littoral periwinkles 

 (Littorina), which show reduction of branchiae and development of a 

 vascularized mantle epithelium, especially in those species occurring above 

 mid-shore. Nerita and Acmaea are other littoral prosobranchs which have 

 vascularized mantle-epithelia. Sea-slugs such as Ancula found in the tidal 

 zone possess rather rigid branched gills. Some pulmonates have returned 

 to the sea and lead an amphibious existence in the tidal zone. Siphonaria, 

 a limpet-like form, is provided with a vascular epithelium in the roof of 

 the mantle cavity, which appears to be used as an aquatic and aerial lung. 

 In Onchidella there is a lung chamber which subserves respiration when 

 the animal is in air but is closed off when submerged. Pulmonary respira- 

 tion is supplemented by gaseous exchange through the dorsal mantle, 

 which bears gills or papillae; this surface is kept moistened by the secretion 

 of pallial glands when the animal is in air (3, 64). 



Isopods show various respiratory adaptations towards terrestrial con- 

 ditions. A transitional series is afforded by Idotea from the middle shore; 

 Ligia at high-tide mark ; Oniscus, a terrestrial animal which inhabits damp 

 places; and Porcellio, which prefers somewhat drier conditions. Idotea 

 and Ligia shelter in moist niches and become active at night; their pleopods 

 are normal in structure, and act as gills. The pleopods of terrestrial species 

 are thicker than those of aquatic forms, and in some of the former there 

 are capillary grooves for draining water towards the gills and keeping them 

 moist (24). 



Among decapod crustaceans there is a tendency for reduction of the 

 gills as the animals become increasingly terrestrial. This is brought out in 

 Table 4.2, compiled by Pearse, in which species of anomurans and brach- 

 yurans are arranged by habitat. Essentially terrestrial crabs (hermit 

 Coenobita, robber Birgus, ghost Ocypode) have fewer gills than aquatic 

 species. Coenobita tolerates amputation of its gills and in their absence 

 can obtain at least enough oxygen for survival. Reduction in gill area 

 relative to unit body weight is also correlated with migration from sea 

 to land in brachyurans (71). 



Modifications are commonly present in littoral species for retaining 

 water in the gill chamber — by special folds, hairs, etc. (Sesarma, Uea). 

 Reduction of gills is compensated by the development of accessory modes 

 of respiration suitable for terrestrial life. The gill chamber is enlarged and 

 used as a lung in terrestrial crabs (Gecarcinus, Ucd). In Birgus it is divided 

 into an upper lung and a lower gill chamber. Vascularized epithelium 

 (protuberances, tufts) in the branchial chamber, supplementing the reduced 

 gills, occur in many species (Ocypode, Birgus, Uca). The integument of 

 the anterior ventral surface is very vascular in Coenobita, and serves for 

 respiratory exchange in the absence of well-developed gills. Mechanisms 



