RESPIRATORY SYSTEM IN THE INVERTEBRATA. 93 



ceived and emitted from a tube which is ramified in an arborescent 

 manner, with terminal cellules. 



Diplo-neura. — The highly vascular skin of the entozoa performs 

 the function of respiration — in some, through the medium of water ; 

 and in others the blood is oxygenated on the mucous surface of the 

 animals they infest. In the cirrhopoda respiration is performed by 

 the arms and by the leaf-like fringed membranes attached to the 

 anterior part of the sides of the body. The surface of the body is 

 in general the seat of respiration among the annelida, but in the 

 lumbrici, nereides, and hirudines respiration is in part effected by a 

 series of minute membranous sacs under the skin of the abdomen, 

 each having a separate external opening. Respiration is aerial in 

 the greater number of insects, the air being received through a 

 number of stigmata, and carried by the tracheae in some cases into 

 vesicles, and in others into longitudinally ramifying trunks. 



Many insects in their larval state breathe by means of branchiae 

 in the water, and some in their perfect condition breathe water, 

 although they have an internal tracheal apparatus ; from the water 

 in these branchiae the air is separated, and passed in the gaseous 

 state through the ramified tracheae. There is a very curious fact 

 connected with the eristalis, this disgusting insect has the last ring 

 of its body elongated into a membranous tube, within which there 

 is a second horny tube, which the animal can extend to the surface 

 for the purpose of respiration, whilst it lives at the bottom where it 

 procures its food in the filth of sloughs, sewers, and stinking privies. 

 The tracheary aracknida resemble insects in their tracheal tubular 

 respiration, whilst the respiratory organs of the pulmonary arachnida 

 consist of small sacs opening externally, and situated at the under 

 surface of the abdomen. Both sets of organs are enjoyed by the 

 segestria and dysdera. In tracing the progress of development of 

 the respiratory apparatus in the higher Crustacea, as the astacus 

 fluviatilis, it will be found to present four principal periods ; lstly, 

 that which precedes the appearance of this apparatus ; 2dly, that in 

 which the branchiae are not distinguishable from the organs dedi- 

 cated to locomotion or mastication ; 3dly, that characterised by the 

 transformation of these into organs wholly dedicated to respiration, 

 and distinct from the extremities ; and 4thly, that during which the 

 branchiae sink inwards, and become lodged in the branchial cavities. 



By reviewing the respiratory apparatus in the different groups of 

 crustaceans, it will be found that the several stages of development 

 of the higher orders constitute the permanent types of the lower 

 series ; thus, to the first stage of organisation belong the greater 

 number of the entomostraca and copepoda ; to the second, the bran- 

 chiopoda ; to the third, the amphipoda ; and to the fourth the entire 

 order of the decapoda. The greater number of the Crustacea live 

 under water; some, however, as the gecarcini, or land-crabs, con- 

 stantly live out of water, but it is necessary t ! at their ^piratory 

 membrane shall be kept humid, and for this purpose the membrane 

 is thrown into folds in the form of reservoirs for containing water. 



