OF THE PARASITIC ISOPODA. 
515 
The formation of the muscles lias begun in the abdomen and limbs, the cells being 
arranged in rows. 
The sixth stage I have taken is that in which the egg-membranes burst, so that some 
of the embryos are still in the egg, while others are free. 
In the living embryo (fig. 16) most of the organs can be more or less distinctly seen. 
In the abdominal region the heart (fig. 16, H) is very conspicuous and beats 
vigorously. 
The blood enters it by four valvular openings (V) arranged in a spiral, and leaves it 
by five anterior aortse, each of which is provided with valves. 
The blood corpuscles are not very numerous, but there are enough to show the 
course of the circulation, and in those individuals which have escaped from the egg, 
they can be seen passing through the branchiae. 
Round the edge of each branchial plate, between the two chitinous laminae of which 
it is composed, there is a clear space answering the purpose of the afferent and efferent 
vessels of the organ. The central part of the lamella is occupied by a number of cells, 
leaving a meshwork of spaces between them. 
The blood flows up the inner edge of the lamella, and passes across to the outer 
edge to return to the heart. 
The passage of the corpuscles to and from the gill is very rapid, but they often get 
entangled in going from one side to the other and remain in the gill for some time. 
Occupying the greater part of each side of the thorax are the liver coeca (fig. 16 , L). 
They are of a different colour from the yolk and are filled with a fluid in which are 
suspended numerous oil drops. These are kept in constant motion by the contractions 
of the coeca, and sometimes pass from one coeciun to another ; the coeca lie entirely 
above the yolk, and by gently moving the cover-glass can be made to glide about over 
it without in any way injuring the embryo. 
In the second segment behind the head the liver coeca are attached to the fore-gut. 
The arrangement of these parts cannot be made out in the living state, but by cutting 
the embryo in half so as to allow the yolk to escape without injuring the liver coeca, 
it is possible to see the duct, and by gently pressing the cover-glass one can make 
the oil drops pass from the liver into the fore-gut. Fig. 17 is an optical section 
of these parts. Two projections bearing hairs may be seen in the front part of the 
stomach. 
The liind-gut can be traced from beneath the heart, in the abdomen, to its junction 
with the fore-gut, although from its transparency it is not at first very easily seen. 
Like the liver, it lies entirely above the yolk. For the greater part of its course it is 
very narrow, but at the anterior end it appears to dilate a little before joining the fore- 
gut; its walls at this point are extremely thin. At a rather later stage, when the 
yolk has decreased to about half its present size, it is quite easy to tease out the 
embryo, so as to see the junction of the fore- and hind-guts and liver coeca. 
The yolk at this stage is surrounded by a membrane (Dottersack) continuous with 
