SEA-FISHERIES LABORATORY. 925 
delicate fibrous tissue pass from wall to wall, and 
which contains blood corpuscles. There are 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 trabecule cross 
the cavities of these vessels. It 1s 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 
sreater 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 renal organ, and having under- 
