COMPAEATI\'E STCDY OF SCOMBROID FISHES. 375 



the scpnncnt of the fifth vortebm, in Paralhv.mivs aud KerAhunnus iu the stv^rmcut 

 i)f the eighth or iiiuth vortobrn. EsCHRICHT oljseivml that the dorsjil aortii liecomes 

 abruptly sknckr after the riimificatiou of tlu se cutaneous arteries, m the follow- 

 ing words : — " Nach dem Abgang der uiienae aociUareä wird die Aortii plötzlich 

 mehr als um die Hälfte dünner im Durchmesser ". 



In the Thuuuidat> tlii' (ut;ine<jus arteiy runs obliqueh' backwai'd and 

 dorsalward, pixssLug 1)ehind the third {Tliunnns) or fifth lib {Paraihwinus and 

 Keot/iuntim), and reaches the surface of the 1x>dy before the intenuuscular bone, 

 attiichcd al»ve the itiot of the respective rib. Before reaching the surface of 

 the body each arteiy is spht into two equal branches, running dorsal and 

 venti-al to the lateral median hue, nearl}' parallel to each other (fig. 3). They 

 are united again in the caudal iwrtion by a ti'ausverse commissure, and the 

 comim'ssure is again united to the doi-sjil aorta hj a piir of horizontal 

 s^mental arteries (fig. 4). Each branch of the cutaneous aiieries gives oflf, at 

 the sm-fiice of the body, segmental arteries obHquely backwards along the borders 

 of myotomes for some distance and then bends forwards. The dorsal branch 

 sends dorsal sqjmi utal arteries only, and the venti'al branch venti'al segmental 

 arteries oulj-. Tliese cutaneous segmental arteries send aiieiioles axially, 

 along myooommatii or straightly inward (fig. 3). Both dorsal and venti-al 

 cutaneous arteries, moreover, send one or t^vo rows of very numerous parallel 

 arterioles, quite close to each other. These arterioles run obliquely inward, 

 along the Ijounckiry lietweon the red and dark red portions of the lateral 

 muscle. They are soon divided into several smaller canals and always run in 

 association with similar venules making a membranous sheet investing and 

 nourishing the dark red porti<JU of the lateral muscle. The arterioles in the 

 vascular sheet gradually unite agsiin reduced in number at about midway 

 between the surface and the axis of the body, and vanish in capillaries, so 

 that the dark red portion of the Literal fbuscle is not entirely covered with 

 fine bloodvessels and cfipillarics u&vc the axijil i^art. The vascular sheet is 

 thick near the sur&ice of the body, becoming gradually thin as it approaches 

 the axis of the body. In the Plecostei, except Evthynnus, the cutaneous 

 arteries always run on the axial and inner side of the accompanying veins. 

 Generally the axial margin of the cufameous vein pirtly covers or is at least 

 apposed to the abaxial margin of the accompanying cutiuieoiis artery ; but in 



