444 ANIMAL RESPIRATION 



The familiar land crustaceans known as Wood-Lice, common 

 under stones and in the crevices of walls, present another instance 

 of the same sort of thingr. We here find that the limbs of the 

 abdomen are used in breathing, and each of them consists in many 

 cases of an outer and inner plate. The latter is delicate in texture, 

 and does the respiratory work, while the outer plates are protec- 

 tive, and also prevent desiccation. The latter point is one of 

 some importance, for it is probable that all sorts of wood-lice 

 require damp air for breathing purposes. We have, in fact, a 

 condition which is half-way between ordinary air-breathing and 

 the state of things characteristic of thorough-going aquatic forms, 

 which use the air dissolved in the water by which they are sur- 

 rounded. It may also be added that in some kinds of Wood- Lice 

 the outer plates of the abdominal limbs are hollowed out by 

 air-cavities, especially in certain cases where the inner plates have 

 altogether disappeared. 



LAND NEMERTINES (Nemertea), EARTHWORMS AND 

 LAND LEECHES (Annelida), AND LAND PLANA- 

 RIANS (TURBELLARIA) AS AIR-BREATHERS 



None of the forms mentioned in the above heading possess 

 special breathing organs, but respire by means of the skin, which 

 is unable to perform its work unless kept moist. This is effected 

 for the most part by means of a slimy substance secreted by the 

 epidermis, just as in Frogs (see p. 423), which breathe partly in 

 this way. It also follows that these various terrestrial forms can 

 only live in damp places, being even more dependent upon a 

 humid atmosphere than Wood- Lice (see above). 



Land Nemertines have been observed in widely separated 

 parts of the world. One very interesting form {Geonevicrtes 

 Palaensis) was discovered by Semper in so remote a locality as 

 the Pelew Islands of the Pacific, where it lives among fallen 

 leaves and the roots of trees. 



Earthworms. — When animals which possess a distinct blood- 

 system breathe by means of the skin, that organ is always richly 

 supplied with blood, an arrangement which is obviously favourable 

 to absorption of oxygen and elimination of carbonic acid gas from 

 the system. The Earthworms furnish a case in point, beside 

 which the skin is not only of slimy nature, but there is also a 



