Two Types of Dorsal Vessel 75 



uniform tube, encircled by innumerable and close -set 

 muscle-cells. The cells are usually deficient above and 

 below, or united by non-muscular substance, so that they 

 do not form complete circles about the dorsal vessel, but 

 pairs of semicircles. The hinder part of such a tube may 

 afterwards enlarge and form a heart, whose simple 

 muscle-cells are often replaced by strands of striated 

 muscle, as in the larger Chironomus-larvae. In the rest 

 of the tube a great increase of length takes place with- 

 out increase or even with considerable diminution in 

 the number of the muscle-cells, which therefore become 

 widely spaced. Certain of the muscle-cells become much 

 enlarged, and send out nucleated projections into the 

 cavity of the dorsal vessel. A pair of such projections, or 

 in particular cases a single projection, forms a simple 

 cellular valve, which, when the muscle contracts, prevents 

 the passage of the blood. Such cellular valves are nearly 

 always opposite, but in the dorsal vessel, or some part of 

 it, of the larvae of Corethra, Ptychoptera, and Calliphora, 

 they are not opposite, but alternate ; in the Corethra- 

 larva (where they are found only in the last chamber) 

 they seem to be multicellular, but are not really so. 

 Between two pairs of cellular valves, ostia, or inlets 

 for the blood, may form; these too are specially associated 

 with muscle-cells, and nuclei are often visible, one just in 

 front and another just behind the inlet. The cellular 

 valves and ostia often show something of a segmental 

 arrangement, which is however usually effaced in the 

 aorta and may disappear altogether. 



In Dipterous larvae two types of dorsal vessel have Two types 

 been described. In the first type, which is by far the vessel. 

 commonest, both in Diptera and in insects generally, 

 there is no important difference of structure between the 

 heart and the rest of the abdominal dorsal vessel, which 

 is cantractile throughout, and provided with several pairs 

 of inlets ; of the many pairs of muscle-cells one pair here 

 and there becomes enlarged, and forms cellular valves, 

 whose free surface is often lobed ; these cellular valves 

 are intermediate between the inlets, and generally nearer 

 to the one behind than to the one in front. There may 

 be no enlargement of the hinder part of the dorsal vessel, 

 and striated muscle-fibres are not found ; aortic valves 

 may be present or absent \ 



' Jaworowski says that in Dipterous larvae which exhibit this type of 



