PROFESSOR OWEN ON LIMULUS POLYPHEMUS. 189 



* apodemata ' or inflections of the ventral crust (PI. XXXVI, fig. 1, n — vi), forming, or 

 rising from, the articular cavities in the cephaletral plastron for those limb segments. 1 



" The ' levatores telsiy if I may be permitted to Latinise Spence Bate's term for the 

 ' tail-spine,' rise from the upper median lateral parts of the inner surface of the thora- 

 cetron ; the median fasciculi form one elongated muscle (PL XXXIV, fig. 1, m 7), which 

 is inserted into the upper basal process, and directly tends to raise the spine ; there are 

 two shorter lateral masses (id., m 8), converging to be inserted into the same process, 

 but which, if acting independently, would draw the spine outwards as well as upwards. 

 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 tehi. — Two shorter and broader but powerful muscles (id., 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 

 action, depress the tail-spine ; their lateral portion, combining with the corresponding 

 one of the levator tehi, draws the spine to that side. When the spine was fixed the 

 muscles would act as flexors, extensors, or adductors 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 (PI. XXXIV, fig. 1, ix — xm), 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 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 (PI. XXXV, 

 fig. 6, m 11). 



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

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

 levator action upon the branchipeds with that of the ' depressores t/ioracetri.' 



" In the cephaletral limbs (in — vi, Pis. XXXIV and XXXV) the haunch-joint 

 1 J. vander Hoeven, op. cit., pi. iii, fig. 10, b, c. 



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