MEROSTOMATA. 



547 



the cephalic buckler comes a second, somewhat rhomboidal shield, 

 the segments of which are immovably welded together, and which 

 is usually regarded as representing the abdomen, though some 

 authorities consider it as the thorax. The lateral margins of 

 this abdominal shield carry movable spines, and on its under sur- 

 face in front are placed the generative openings, protected by a 

 broad, divided " operculum," which is formed by the first pair of 

 abdominal limbs. The oper- 

 culum not only covers the 

 reproductive apertures, but 

 more or less extensively con- 

 ceals the remaining five pairs 

 of abdominal limbs. These 

 latter are much modified, and 

 support a number of delicate 

 lamellae arranged like the 

 leaves of a book, which act 

 as branchiae. Lastly, the ab- 

 dominal shield has movably 

 articulated to its hinder mar- 

 gin a long ensiform " telson " 

 (fig. 410). 



The integument of Limu- 

 lus is thoroughly hardened by 

 the deposition of chitine, and 

 forms a resisting shell. Thin 

 sections show that this is 

 largely composed of a pecu- 

 liar finely tubulated tissue, 

 which has very much the ap- 

 pearance of dentine under the 

 microscope (fig. 411, a). In 

 the inner layer of the shell, 

 the minute tubuli just men- 

 tioned are accompanied by a 

 limited number of vertical 

 canals of larger size (fig. 411, 

 a, c), the structure thus com- 

 ing closely to resemble that shown in vertical sections of the shell 

 of the Trilobites. 



The embryo of Limulus (fig. 382) is destitute of the tail-spine of 

 the adult, this structure only existing in a rudimentary form ; and 

 in this stage of its existence, as previously noticed, it bears a strong 

 resemblance to certain of the Trilobites. 



It is not necessary here to enter into the vexed question as to 



Fig 



410. — Limulus moluccanus, viewed from the 

 dorsal aspect, and reduced in size. (Recent.) 



