THE EVIDENCE OF THE RESPIRATORY APPARATUS 181 
for the dorsal aorta cannot by any possibility represent that 
heart. 
Although it is not now functional the original existence of so 
important an organ as a dorsal heart may have left traces of its 
former presence ; if so, such traces would be most likely to be visible 
in the lowest vertebrates, just as the median eyes are much more 
evident in them than in the higher forms. In Fig. 58 the position of 
the dorsal heart is shown in Limulus, and in Fig. 70 the shape and 
extent of this dorsal heart is shown. It extends slightly into the pro- 
somatic region, and thins down to a point there, runs along the length 
of the animal and finally thins down to a point at the caudal end. 
The heart is surrounded by a pericardium, from which at regular 
intervals a number of dorso-ventral muscles pass, to be inserted into 
the longitudinal venous sinus on each side. These veno-pericardial 
muscles are absolutely segmental with the mesosomatic segments, 
and are confined to that region, with the exception of two pairs in the 
prosomatic region. Their homologies will be discussed later. 
Any trace of a heart such as we have just described must be 
sought for in Ammoccetes between the central nervous system and 
the mid-line dorsally. Now, in this very position a large striking 
mass of tissue is found, represented in section in Fig. 73, f. It 
forms a column of similar tissue along the whole mid-dorsal region, 
except. at the two extremities; it tapers away in the caudal region, 
and headwards grows thinner and thinner, so that no trace of it is 
seen anterior to the commencement of the branchial region. It 
resembles in its dorsal position, in its shape, and in its size a dorsal 
heart-tube such as is seen in Limulus and elsewhere, but it differs 
from such a tube in its extension headwards. The heart-tube of 
Limulus ceases at the anterior end of the mesosomatic region, this 
fat-column of Ammoccetes at the posteriorend. In its structure there 
is not the slightest sign of anything of the nature of a heart; it is 
a solid mass of closely compacted cells, and the cells are all very 
full of fat, staining intensely black with osmic acid. Nowhere else 
in the whole body of Ammoccetes is such a column of fat to be found. 
It is not skeletogenous tissue with cells of the nature of cartilage- 
cells, as Gegenbaur thought and as Balfour has depicted (Vol. IL., 
Fig. 315) in his ‘ Comparative Embryology,’ as though this tissue were 
a part of the vertebral column, but is simply fat-cells, such as might 
easily have taken the place of some other previously existing organ. 
