THE EVIDENCE OF THE RESPIRATORY APPARATUS I J J 



branchial appendage of Ammoccetes at right angles to the carti- 

 laginous branchial bar. 



Further, the observations of Blanchard, Milne -Ed wards, Kay 

 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 reinarque 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 svstem. 

 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-lamella3 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-lamellaj 

 the blood is collected into an efferent or branchial vein (v. br.), which 



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