No. I.] STUDIES ON LIMULUS. 113 



The abdomen is extended by two pairs of powerful intej'-tergal 

 muscles, Pis. VIII and IX, Figs. 4 and 5, i.m. (only the external 

 ones are represented) in the median line on the haemal side of 

 the animal. The external pair of muscles arise from the median 

 faces of the first pair of entapophyses, and the internal pair 

 from an extended area upon the haemal, median portion of the 

 carapace. Both pairs are inserted in the median line on the 

 anterior, haemal border of the abdominal carapace. 



Seven pairs of haemo-neitral viuscles (Pis. VI, VIII, and IX, 

 Figs. I, 4-6, h.n.m.^~''f) arise from the haemal side of the 

 abdominal carapace. The first six pairs [h.n.m.^'^^) are inserted 

 upon the six abdominal endochondrites, and hence belong to 

 the opercular and five branchial metameres, respectively. The 

 seventh pair (Ji.n.mJi) are inserted upon the neural side of the 

 carapace posterior to the last appendage. The first pair 

 (Ji.n.vi.^), which belong to the opercular metamere, arise upon 

 the anterior border of the abdominal carapace, from the median 

 side of a pair of small protuberances which may possibly rep- 

 resent the remnants of a pair of entapophyses, though it is 

 more likely that the entapophyses of the chilarial and opercular 

 metameres have fused with each other. As will be seen later, 

 the fact that there is a fusion of other structures in this region 

 serves to support this hypothesis. The remaining six pairs of 

 haemo-neural muscles {li.n.viy'f) arise from the carapace on the 

 median side of the six pairs of abdominal entapophyses. 



The haemo-neural muscles aid in holding the endochondrites 

 in place by counteracting the contractions of the internal bran- 

 chial muscles (i.b.in.) which arise from the endochondrites. 

 They also serve to compress the abdomen. 



Eight pairs of veno-peiicardiac muscles (Text-fig. 4 ; Pis. 

 VIII and IX, Figs. 4 and 6, v.p.mP~'^), "brides transparentes," 

 of Milne-Edwards, are attached to the neural side of the peri- 

 cardium opposite the eight pairs of ostia {os.^'^) of the heart. 

 These muscles pass neurally on each side of the intestine to the 

 integument upon the neural side of the body. Instead of being 

 attached directly to the integument, however, the bases of the 

 muscles expand and become continuous with a tough connective- 

 tissue membrane running longitudinally between the branchio- 



