526 MR. W. B. S. BENHAM ON THE MUSCULAR 
No. 20. The External Branchial Muscles.—Inserted in the anterior lamella of the 
genital operculum is a large muscle which passes at first upwards (Pl. LXXV. fig. 1); 
then, when it leaves the appendage, it passes outwards, backwards, and upwards, out- 
side the other muscles of the metasoma to its attachment on the anterior border of the 
mesosomatic carapace, outside the line formed by the bases of the entapophyses 
(Pl. LXXIV. fig. 1, and Pl. LXXVI. fig. 1). 
There is a similar pair of muscles in each of the other five mesosomatic appendages. 
[These are attached at their tergal origins near the corresponding entapophysis 
(Pl. LXXYV. fig. 7). There is no entapophysis to the tergum of the genital segment, as 
is rendered obvious by the position of this muscular attachment, unless we may consider 
the great prosomatic entapophyses as originally belonging to that segment, but trans- 
ferred and ankylosed to the prosoma, just in the same way as the body of the atlas 
vertebra of mammals is transferred to the axis.—E. R. L.] 
No. 21. Anterior Entapophysio-branchial Muscles——Nearer the middle line than 
no. 20 is a smaller muscle inserted in the anterior lamella of the genital operculum, 
and passing nearly directly upwards, and slightly outwards, outside all the other 
muscles of the mesosoma but no. 20, to its attachment to the great prosomatic entapo- 
physis (see Pl. LX XIV. fig. 1). 
A: similar muscle occurs in each of the five succeeding mesosomatic appendages, 
each attached to the entapophysis of its own segment. 
No. 22. Posterior Entapophysio-branchial Muscles.—From the posterior lamella of the 
genital operculum there goes a muscle upwards and forwards, beneath the muscle 1, 
to its attachment on the prosomatic carapace, alongside the sixth coxo-tergal muscle, 
nearer the median plane. 
In each of the five succeeding mesosomatic appendages is a similar muscle, but each 
is attached to the entapophysis of the preceding segment; thus this muscle from the 
second mesosomatic appendage is attached to the great prosomatic or first entapophysis, 
that from the third to the second entapophysis, and so on. 
No. 23. Pre-entapophysio-branchial Muscles.—In the second and following mesoso- 
matic appendages is a second muscle, attached in each case to the entapophysis of the 
preceding segment, but inserted into the anterior instead of the posterior lamella of 
the appendage. 
No. 65. Chilarial Muscles.—A small muscle passes from the posterior process of the 
prosomatic entochondrite into the chilaria. 
III. Muscles of the Appendages. 
a. Prosomatic Appendages. No. 24. The Tergo-coral Muscles of the first Pair.— 
From the coxa of the first prosomatic appendage a small muscle rises nearly vertically, 
passing just across the inner border of the anterior cornu of the prosomatic entochon- 
drite, and between the muscles of this process to the carapace, to which it is attached 
