STUDIES ON LIMULUS. 



EXPLANATION OF PLATE I. 



23 



Fig. I. X 2. Endosternite of Limulus from neural side, anterior extremity 

 towards top of plate. Only the bases of the lateral cornua (/. c.) are represented. 

 The anterior cornua (a. c.) are prolongations of the thickened sides of the plas- 

 tron. The sides of the plastron are roughened by muscular attachment. Inside 

 the marginal ridge {m. r.) the floor of the plastron anteriorly is smooth and 

 marked only by the slight transverse furrows indicating the course of the fibers 

 of the cartilage, for there are no muscles attached here except a few strands 

 going to the oesophagus. Posterior to this space the irregular markings indicate 

 an area of muscular attachment, the anterior limit of which is marked by a V- 

 shaped outline. The marginal ridge {m. r.) runs along each side of the plas- 

 tron, from the anterior cornua to the occipital ring {oc. r). Anteriorly the ridge 

 rises sharply above the floor of the plastron, but posteriorly it is only a low, 

 rounded shoulder. 



The occipital ring {oc. r.), shaded lighter than the rest of the plastron, is marked 

 on the anterior margin by the attachments of muscle fibers going to the integu- 

 ment behind the mouth. On the top are two shallow pits (ch. m.) in which a 

 pair of muscles going to the chilaria take their origin. From the posterior side 

 of the ring thin strands of connective tissue go to the capsuliginous bars {cap. b.). 

 The capsuliginous bars {cap. b.) arise from t|;ie thinner portion of the plastron, in 

 the angle between the posterior process {p-pr.) and the latero-posterior processes 

 {I. p. pr.). The posterior process {p. pr.) is unpaired and is divided into two 

 divisions. 



The first pair of foramina (/.') are represented by black dots at the bases of the 

 latero-posterior processes in the thick portion of the cartilage ; the second pair 

 {/■^) by the light circles in the thin portion back of the occipital ring. 



Fig. 2. X 2. Endosternite of Limulus from haemal side. The distal ends of 

 the lateral cornua upon the left side are disconnected from the proximal portions. 

 These cornua are fully one-third longer than here represented. The drawing 

 shows their mode of attachment to the haemal side of the plastron and the 

 courses of the fibers in and about their bases. This portion of the haemal sur- 

 face is free from muscles. The area of muscular attachment begins with the 

 splintery appearance just anterior to the haemal processes {k. pr.). The edges of 

 the plastron between the lateral cornua and the haemal processes are slightly 

 elevated. The haemal processes {h.pr.) are stout and rise a considerable distance 

 above the body of the plastron. 



The first pair of foramina (/.') appear as dark slits a short distance behind the 

 haemal processes. 



The posterior process {p-pr.) begins as a low ridge in the median line opposite 

 the haemal processes. Each division of its forked end is deeply grooved. The 

 capsuliginous bar {cap. b.) on the left is shown entering the base of one of the 

 chilaria {ch.), to the posterior side of which it is attached. Running along the left 

 side of the plastron is a connective tissue membrane {mem.) to which the veno- 

 pericardiac muscles are attached. The bases of the two anterior of these muscles 

 {v. p. w.'-^) are represented. Neural to the base of the second muscle {v. p. m.^) 

 the membrane is attached to the integument {t7tt.) by a bundle of connective tissue 



