THE EVIDENCE OF THE RESPIRATORY APPARATUS I 7 I 



The paper by Benhani gives a full description of the musculature 

 of Limulus, and according to his arrangement the muscles are 

 divided into two sets, longitudinal and dorso-ventral. Of the 

 latter, each mesosomatic segment possesses a pair of dorso-ventral 

 muscles, attached to the mid-ventral mesosomatic entochondrite, and 

 to the tergal surface (Fig. 58, Dv.). These muscles are called by 

 Benham the vertical mesosomatic muscles. I shall call them the 

 somatic dorso-ventral muscles, in contradistinction to the dorso- 

 ventral muscles of the branchial appendages. Of the latter, the two 

 chief are the external branchial (Fig. 66, m 3 ) "and the posterior 

 entapophysio-branchial (Fig. 66, m{) ; a third muscle is the anterior 

 entapophysio-branchial (Fig. 66, m 2 ). Of these muscles, the posterior 

 entapophysio-branchial (mi) is closely attached along the branchial 

 cartilaginous bar up to its round-headed termination on the anterior 

 surface of the appendage. The anterior entapophysio-branchial 

 muscle (m 2 ) is attached to the branchial cartilage near the 

 entapophysis. 



In the case of the scorpion, as described by Miss Beck, the 

 branchial appendage has become reduced to the branchiae, and the 

 intrinsic appendage-muscles have entirely disappeared, with the 

 possible exception of the small post-stigmatic muscle ; on the other 

 hand, the dorso-ventral somatic muscles, which are clearly homolo- 

 gous with the corresponding muscles of Limulus, have remained, and 

 become the essential respiratory muscles. 



Of these two possible types of respiratory movement it is quite 

 conceivable that in the water-breathing scorpions of olden times 

 and in their allies, the dorso-ventral muscles of their branchial 

 appendages may have continued their role of respiratory muscles, and 

 so have given origin to the respiratory muscles of the ancestors of 

 Ammoccetes. 



The respiratory muscles of Ammoccetes are three in number, and 

 have been described by Nestler and Miss Alcock as the adductor 

 muscle, the striated constrictor muscle, and the tubular constrictor 

 muscle (Fig. 65, m. add,, m.c.s., and m.c.t.). Of these, the constrictor 

 muscle (Fig. 71, m. con. str.) is in close contact with its cartilaginous 

 bar, while the adductor (Fig. 71, m. add.) is attached to the cartilage 

 only at its origin and insertion, and the tubular muscles (Fig. 71, 

 m. con. tub.) have nothing whatever to do with the cartilage at all, 

 being attached vent-rally to the connective tissue in the neighbourhood 



