The difference between the movements of respiration in Limulus and those of the scorpions consists in the fact that, although in both cases respiration is effected mainly by dorso-ventral muscles, these muscles are not homologous in the two cases: in the former, the dorso-ventral appendage-muscles are mainly concerned, in the latter, the dorso-ventral somatic muscles. {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, _m_3_) and the posterior entapophysio-branchial (Fig. 66, _m_1_); a third muscle is the anterior entapophysio-branchial (Fig. 66, _m_2_). Of these muscles, the posterior entapophysio-branchial (_m_1_) 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 branchiæ, 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 homologous 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 _rôle_ of respiratory muscles, and so have given origin to the respiratory muscles of the ancestors of Ammocoetes. The respiratory muscles of Ammocoetes 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 {172}of the ventral aorta (_V.A._), and dorsally to the mid-line between the dorsal aorta (_D.A._) and the notochord.