470 PROF. OWEN ON THE ANATOMY 
of the above entosternal muscles, their action would be to retract and raise the 
entosternon. 
The functions of these * levatores entosterni, in relation to the fixation of the endo- 
skeleton, are more especially in opposition to muscles arising from its lower and lateral 
parts to be inserted into basal entapophyses of the five posterior pairs of cephaletral 
limbs. But the principal muscles acting on the basal joints of these members arise from 
the * apodemata,' or inflections of the ventral crust (Pl. XXXIX. fig. 1, 11-v1), forming, 
or rising from, the articular cavities in the cephaletral plastron for those limb- 
segments *. 
The * levatores telsi” (Pl. XXXVI. fig. 2, u, u), if I may be permitted to latinize 
Spence Bate's term for the * tail-spine,’ rise from the upper median lateral parts of the 
inner surface of the thoracetron; the median fasciculi form one elongated muscle 
(Pl. XXXVII. fig. 1, m7), which is inserted into the upper basal process, and directly 
tends to raise the spine: there are two shorter lateral masses (ib. ms) converging to be 
inserted into the same process, but which, if acting independently, would draw the spine 
outward as well as upward. Both median and lateral muscles acting together would raise- 
the spine forcibly, or if the spine were the fixed point, and the thoracetron depressed at 
an angle therewith, would tend to raise that part. 
Depressores telsi.—Two shorter and broader but powerful muscles (ib. m 9), having 
the double oblique or penniform disposition of fibres, rise from the lower terminal part 
or segment of the thoracetron (ib.), and converge to be inserted into the sides of the 
basal entapophyses from below the articular condyles of the tail-spine. "These, combining 
in aetion, depress the tail-spine; their lateral portion, combining with the corresponding 
one of the levator telsi, draws the spine to that side. When the spine was fixed the 
muscles would act as flexors, extensors, or abductors of the thoracetron. 
Muscular fasciculi for the protraction and retraction of the thoracetral appendages 
rise from the apodemata of that division of the body. 
Protractores branchipedum.—The limb-plates of each lateral moiety of the broad gill- 
bearing lamella has two principal muscles: one, arising from the outer part of the 
apodeme in advance, subdivides into fasciculi, which descend, penetrating the fore or 
under surface of the gill-limb (PL XXXVII. fig 1, m 10), and radiate therein to be 
attached to the several segments. These fibres protract the limb, change its recumbent 
for the erect position, and in that movement separate the gill-plates and facilitate the 
flow of water through their interspaces. 
Retractores branchipedum.—These muscles rise from the base of the apodeme of their 
own gill-foot, near the place of articulation of the latter, and spread upon the hinder, 
inner, or upper surface of the proximal lamelliform joint before penetrating the interior 
of the succeeding ones. They retract or draw up the gill-feet, approximate and press 
together the gill-plates, and squeeze out the water from their interspaces. The 
insertional fibres of this muscle are shown on the anterior thoracetral lamelliform 
limb, which serves as a cover or ‘ operculum’ to the genital outlets (PL XX XVIII. 
fig. 6, m 1). 
* Van der Hoeven, op. cit. pl. iii. fig. 10, 5, c. 
