THE EVIDENCE OF THE RESPIRATORY APPARATUS l8l 



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 Ammocoetes 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, /. 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 posterior end. 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 Ammocostes 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. II., 

 Fig. 315) in his ' Comparative Embryology,' as though this tissue were 

 a pari of the vertebral column, but is simply fat-cells, such as might 

 easily have taken the place of some other previously existing organ. 



