81 



delicate fibrous tissue pass from wall to wall, and 

 which contains blood corpuscles. There arc no skeletal 

 structures. 



In any section of the wall four cells are very obvious. 

 Two of these are very large, one being situated on each 

 lateral wall. Their nuclei are prominent but stain lightly. 

 There is a very evident striated free border, and the cell 

 bears a number of long and coarse cilia. The two other 

 cells lie nearer the broad end of the filament, and resemble 

 those described. The nuclei are, however, very large and 

 stain intensely, so that they are very evident even under a 

 low magnifying power. Between these two cells the 

 outer wall of the filament is composed of cubical cells 

 bearing short cilia. The inner surface consists of small 

 cubical or even flattened cells. 



Both the vertical afferent and efferent vessels and the 

 inter-filamentar vessels have very thin walls composed of 

 a flattened epithelium. Only a few fine trabeculae cross 

 the cavities of these vessels. It is obvious from a comparison 

 of the area presented by this vascular tissue with the 

 area of the filaments themselves, and from a comparison 

 of the nature of the epithelia in each case that by far the 

 greater part of the gaseous exchange in respiration must be 

 effected through the wall of the vascular tissue proper and 

 not through that of the filaments. The latter, in fact, form 

 a mechanical tissue supporting the series of vascular 

 channels, and by the action of their ciliated epithelium, 

 causing the current of water from without to flow through 

 the bars of the trellis work of each lamella into the supra- 

 branchial cavities. 



The Course of the Circulation. 



The heart is a systemic one. Blood, with the waste 

 products eliminated in the venal organ, and having under- 



