THE EVIDENCE OF THE RESPIRATORY APPARATUS 177 
branchial appendage of Ammoccetes at right angles to the carti- 
laginous branchial bar. 
Further, the observations of Blanchard, Milne-Edwards, Ray 
Lankester, and Benham concur in showing that in both Limulus and 
the scorpion group a striking and most useful connection exists 
between the heart and these two collecting venous sinuses, in the 
shape of a segmentally arranged series of muscular bands (V.p., Fig. 
70 and Fig. 58), attached, on the one hand, to the pericardium, and 
on the other to the venous collecting sinus on each side. These 
muscular bands, to which Lankester and Benham have given the 
name of ‘ veno-pericardial muscles,’ are so different in appearance 
from the rest of the muscular substance, that Milne-Edwards did not 
recognize them as muscular, but called them ‘brides transparentes.’ 
Blanchard speaks of them in the scorpion as ‘ligaments con- 
tractiles, and considers that they play an important part in assisting 
the pulmonary circulation ; for, he says, “en mettant a nu une portion 
du cceur, on remarque que ces battements se font sentir sur les liga- 
ments contractiles, et determinent sur les poches pulmonaires une 
pression qui fait aussitot refluer et remonter le sang dans les vaisseaux 
pneumocardiaques.” Lankester, in discussing the veno-pericardial 
muscles of Limulus and of the scorpions, says that these muscles 
probably contract simultaneously with the heart and are of great 
importance in assisting the flow through the pulmonary system. 
More recently Carlson has investigated the action of these muscles 
in the living Limulus and found that they act simultaneously with 
the muscles of respiration. 
Precisely the same arrangement of veno-pericardial muscles and 
of longitudinal venous collecting sinuses occurs in the scorpions. It 
is one of the fundamental characters of the group, and we may fairly 
assume that a similar arrangement existed in the extinct forms from 
which I imagine the vertebrate to have arisen. The further con- 
sideration of this group of muscles will be given in Chapter IX. 
Passing now to the condition of the branchial blood-vessels of 
Ammoccetes, we see that the blood passes into the gill-lamelle from a 
blood-space in the appendage, which can hardly be dignified by the 
name of a blood-vessel. This blood-space is supplied by the branchial 
artery which arises segmentally from the ventral aorta (V.A.), as seen 
in Fig. 71 (taken from Miss Alcock’s paper). From the gill-lamelle 
the blood is collected into an efferent or branchial vein (v. 07.), which 
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