CIRCULATORY AND RESPIRATORY SYSTEMS. 59 



monly a simple dorsal vessel. Respiration by tracheae, as in 

 Acarus (mite), or by lungs, as in Epeira (true spider). Aera- 

 tion seen in Emydium (water-bear). 



Insecta and Myriapoda. Blood-vessel system incomplete. 

 Dorsal vessel, or heart, articulated. Venous canals com- 

 municate in a dorsal sinus. Respiration trachea!. False 

 tracheae in aquatic, larval, and pupal forms, as in Ephemera 

 (May -fly); but true tracheae in all adult forms of Insecta. 



VERTEBRATA. Blood-vessel system complete ; hepatic portal 

 circulation constant, blood contributed to it from tail in 

 Pisces, and from tail and posterior extremities and kid- 

 neys in certain Batrachia and Ophidia. Heart, with excep- 

 tion of Amphioxus (lancelet), of more than a single cavity. 

 Coloration of blood due to presence of red blood corpuscles. 



Pisces. The venous blood is conveyed to a large sinus 

 near pectoral region. Heart composed of two chambers, a 

 single auricle and ventricle, and is placed at the junction of 

 head and trunk above the position of coracoid bones. It 

 contains venous blood which is sent through branchiae to be 

 returned arterialized to the aorta, and is thence distributed 

 over the body: it is thus seen to be a branchial heart as op- 

 posed to the systemic heart of lamellibranchiate and gas- 

 teropod molluscs. The commencement of the aorta is fur- 

 nished with a special dilatation (bulbus arteriosus) which is 

 rarely absent, as in marsipobranchiates, and Amphioxus. In 

 osseous fishes it exhibits a single row of valves ; but in the 

 cartilaginous, excluding the types already mentioned, several 

 rows are present. The blood-vessel system of Amphioxus 

 differs from that of other vertebrates in the absence of a 

 valvular heart, propulsion of the blood-current being induced 

 through the agency of a number of contractile chambers de- 

 veloped at different points upon the arteries and veins. There 

 are from twenty-five to fifty of these bulbs on either side. 



Branchiae are arranged in all fishes, in connection with the 

 convex surfaces of osseous or cartilaginous arches placed on 

 either side of the throat, in double rows. In Amphioxus, 

 these rows, from seventy to eighty in number, are contained 

 within a chamber opening anteriorly by the mouth and pos- 



