12 



skeleton, are more especially in opposition to muscles arising from its lower and lateral 

 parts to be inserted into basal entapopbyses of the five posterior pairs of cepbaletral 

 limbs. But the principal muscles acting on the basal joints of these members arise 

 from the ' apodemata,' or inflections of the ventral crust (PI. V. fig. 1, ii-vi), forming, 

 or rising from, the articular cavities in the cepbaletral plastron for those limb- 

 segments *. 



The ' levatores telsi' (PI. I. 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 II a. 

 iig. 1, m 7), which is inserted into the upper basal process, and directly tends to raise 

 the spine : there are two shorter lateral masses {ib. m s) 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 bvit 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 entapopbyses from below the articular condyles of the tail-spine. These, combining 

 in action, 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 apodeinata of that division of the body. 



Frotractores 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 (PI. II A. 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. 



Betractores brancliipedmn. — These muscles rise from the base of the apodeme of their 

 own giU-foot, near the place of articulation of the latter, and spread upon the hinder, 

 inner, or upper surface of the proximal lamelliforin joint before penetrating the interior 

 of the succeeding ones. They retract or draw up the gill-feet, approximate and press 

 too'ether the gill-plates, and squeeze out the water from their interspaces. The inser- 

 tional fibres of this muscle are shown on the anterior thoracetral lamelliform limb, which 

 serves as a cover or ' operculum ' to the genital outlets (PI. IV. fig. 6, m u). 



Some small fasciculi, combining their insertions with the proper muscles of the 

 branchipeds, have attachments to the thoracetral entapopbyses, and seem to combine a 

 levator action upon the branchipeds with that of the ' depressores thoracetri.' 



In the cepbaletral limbs (iii-vi, Pis. II. and III.) the haunch-joint {coxa, 1) is of great 



* Van der Hoeveii, op. cit. pi. iii. fig. 10, 6, c. 



