No. I.] STUDIES ON LIMULUS. g 



It is especially noteworthy that the capsuliginous bars differ 

 histologically from the rest of the plastron, and that they are 

 united with the thinner portions of the plastron by what appears 

 to be a true joint. The body of the plastron is composed of 

 fibroid cartilage, while the bars are of capsuliginous cartilage, 

 exactly like that found in the abdominal appendages, the histol- 

 ogy of which has been described by Gegenbaur ('58) and Lan- 

 kester ('84) and more recently by Gaskell ('97). 



8. T/ie Occipital Ring (oc.r., Figs, i, 3, and 4). — At the 

 points where the marginal ridges (nt.r., Fig. i) meet the bases 

 of the latero-posterior processes, two outgrowths are formed 

 which are united with each other distally on the neural side of 

 the ventral cord by a connective tissue membrane. At their 

 bases the processes are slender, but distally they enlarge and 

 thicken, and are joined to the capsuliginous bars {cap.b.) by 

 strands of connective tissue. 



That the occipital ritig or subneural arch thus formed is a 

 true cartilaginous ring is beyond doubt, for serial sections of 

 a young animal through this region show the continuity of the 

 cartilaginous tissue entirely around the cord (Fig. 4). Upon 

 the neural side of the ring are two depressions {ch.m., Figs, i, 

 3, and 4), to which are attached a pair of muscles going to the 

 insides of the chilaria. From the anterior side of the ring, 

 muscle strands pass forward to the integument immediately 

 behind the mouth. 



9. Accessory Structures. — Besides the processes and muscle 

 attachments above mentioned, there are found along the late- 

 ral edges of the plastron on the haemal side, and attached to 

 it by connective tissue fibers, tough membranes {mem., Fig. 

 2), to which are attached the " veno-pericardiac muscles" of 

 Lankester, or the "brides transparentes " of Milne-Edwards 

 (y.p.m.'^-^, Fig. 2). Anteriorly this membrane springs from 

 the side of the plastron just back of the lateral cornua. About 

 midway between the lateral cornua and the haemal process it 

 affords attachment to the anterior or first veno-pericardiac mus- 

 cle (y.p.m.^, Fig. 2). Just anterior to the latero-posterior proc- 

 ess it gives attachment to the second veno-pericardiac muscle 

 {v.p.m.'^. Fig. 2), and at the same point sends a bundle of con- 



