CRAYFISH. 291 
several smaller apertures, which admit of entrance but not 
of exit. 
The blood contains amceboid cells, and the fluid or 
plasma includes a respiratory pigment, hemocyanin (bluish 
when oxidised, colourless when deoxidised), and a lipochrome 
pigment, called zoonerythrin. Both of these are common 
in other Crustaceans. 
Respiratory system.—Twenty gills—vascular outgrowths 
of the body wall—lie on each side of the thorax, sheltered 
by the flaps of the shield. A current of water from behind 
forwards is kept up by the activity of the baling portion, 
or scaphognathite, of the second maxilla. Venous blood 
enters the gills from the ventral sinus, and purified blood 
leaves them by the six channels leading to the pericardium. 
Observed superficially, the gills look somewhat like 
feathers with plump barbs, but their structure is much more 
complex.. The most important fact is that they present a 
large surface to the purifying water, while both the stem 
and the filaments which spring from it contain an outer 
canal continuous with the venous sinus, and an inner canal 
communicating with the channels which lead back to the 
pericardium and heart. 
Three sets of gills are distinguishable. To the basal joints of the 
six appendages, from the second maxillipede to the fourth large limb 
inclusive, the podobranchs are attached. They come off with the 
appendages when these are pulled carefully away, and each of them 
- bears, in addition to the feathery portion, a simple lamina or epepodzie. 
The membranes between the basal joints of the appendages and the 
body, from the second maxillipede to the fourth large limb inclusive, 
bear a second set, the avthrobranchs, which have no epipodites. In 
connection with the second maxillipede there is a single arthrobranch ; 
in connection with each of the five following appendages there are two ; 
so that there are eleven arthrobranchs altogether. There remain three 
pleurobranchs, one on the epimeron of the fifth large limb, and two 
others quite rudimentary on the two preceding segments. ‘The bases 
of the podobranchs bear long setze. 
In Nephrops, the podobranchs are represented by a small rudiment 
on the second maxillipede, and by five well-developed gills on the next 
five appendages; there are eleven arthrobranchs, the most anterior 
being small; and there are four large pleurobranchs. 
Excretory system.—A kidney or “green gland” lies 
behind the base of each antenna, and its opening is marked 
by a conspicuous knob on the basal joint of that appendage. 
