No. I.] STUDIES ON LIMULUS. m 



in a tough fibroid cortical layer. The same kind of cartilage 

 is found in the structures arising from the posterior portion of 

 the endocranium and attached to the insides of the chilaria. 



3. The Muscular System, 



Lankester {Trans. Zool. Soc, London, 1885), with the assist- 

 ance of Mr. Benham, gave a detailed account of the muscular 

 system of Limulus. He, however, omitted the chilarial and the 

 anal muscles, as well as the muscles of the thoracic append- 

 ages, except the coxal muscles. The anal muscles were figured 

 by A. Milne-Edwards {Ann. Sci. Nat., 1873). 



In the cephalothorax the edges and neural surface of the 

 endocranium afford attachment for a great many muscles which 

 radiate to the bases of the ambulatory legs and are inserted 

 upon the inner proximal portions of the coxopodites from 

 the second to the sixth, inclusive. Lankester calls them the 

 plastro-coxal muscles (Text-figs. 2 and 3 ; PI. VII, Fig. 2, 

 j<zand3,^and^, (^«, 3, ^,/, and^^ ^ud corrcspondiug muscles in the 

 second to the sixth appendages). They assist in the compli- 

 cated chewing movements of the mandibles. 



Three pairs of muscles, the tergo-p7'oplastrals (Text-fig. 13 ; 

 Pis. VI, VIII, and IX, Figs, i, 4, and 5, t. p. m. ''-'=) , suspend the 

 anterior cornua of the plastron from the haemal portion of the 

 carapace. 



A small muscle, lateral proplastro-tergal, fastens each of the 

 lateral cornua to the haemal side of the carapace. 



A pair of plastro-tergals (Pis. VIII and IX, Figs. 4 and 5, 

 d.l.p.t.) attach the haemal processes of the endocranium to the 

 carapace, and a pair of plastro-entapopliysials (PI. VIII, Fig. 4, 

 d.i.p.e.) pass from the same processes to the first pair of 

 entapophyses. 



From the posterior portion of the haemal surface of the 

 endocranium large fneso-plastro-entapophysials (PI. VIII, Fig. 4, 

 m.p.e.) also go to the first pair of entapophyses. All of these 

 muscles serve to suspend the endocranium in a firm position in 

 the cephalothorax and to counteract the contractions of the 

 coxal muscles and longitudinal abdominal muscles, which would 



