THE EVIDENCE OF THE RESPIRATORY APPARATUS 171 
. The paper by Benham 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, ms) and the posterior 
entapophysio-branchial (Fig. 66, m1); a third muscle is the anterior 
entapophysio-branchial (Fig. 66, ma). Of these muscles, the posterior 
entapophysio-branchial (7) 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 (mz) 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 branchie, 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 réle of respiratory muscles, and 
so have given origin to the respiratory muscles of the ancestors of 
Ammoceetes, 
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 ventrally to the connective tissue in the neighbourhood 
