23 



converge abruptly to form the terminal duct. The anterior of these receives the tribu- 

 tary ducts of the four chief anterior divisions of the liver ; the posterior terminal duct is 

 formed by the uuion of the same number. The ducts of two or three of the anterior 

 lobes unite to form that which enters the main or terminal anterior duct ; those from 

 the four posterior lobes unite and enter the posterior terminal duct by two canals. The 

 arrangement, however, shown in the subject of fig. 1, PL I., may be varied in other 

 specimens : but the principle of segmental constitution, as here exemplified by the 

 secondary ducts, will be found, I doubt not, in all Linmli. It indicates the liver to 

 have been developed, in relation to the primitive composition of the cephaletron 

 (Cut, fig. 5), of antero-posteriorly succeeding portions, there being a pair of livers or 

 hepatic lobes to each of five or more embryonal segments. The confluence of the ducts 

 interestingly exemplifies the way of subsequent concentrative growth characteristic of 

 the mature and procreative individual. 



The bile is conducted to the intestine by two terminal ducts on each side : the first 

 pah- (fig. 1, PI. I., & fig. 2, PI. II A., I) open upon the sides of the beginning of the tube, 

 where it contracts to the ordinary caHbre ; the second pair (ib. m) open about 9 lines 

 beyond, and nearer the dorsal part of the intestine. 



As in the King-crabs, certain Spiders [Epeira, e. g.) have their ventral mouth t pro- 

 vided anteriorly with a chitinous plate, ' labrum ' or ' prostome,' and posteriorly with a 

 labium or ' metastome,' which is soldered to the cephaletral plastron, not bifid and movable 

 as in LimuUis (Pis. II., II a.. III., & IV. *). The oesophagus rises, at first, vertically 

 dorsad, then bends back at a right angle, traversing in that part of its course the neural 

 ring before expanding into the stomach. This cavity is, in most spiders, produced into 

 caecal appendages, which, in some, extend into the basal joints of the cephaletral limbs. 

 The bile-ducts open into that part of the intestine which traverses the thoracetron 

 (' abdomen ' of arachnologists). The proportion of difference to resemblance must be kept 

 in mind when speculating on the degree of affinity of Xiphosura and Ai-achnida. 



§ 6. Sanguiferous System. — The dissection of Limulus was commenced from that 

 aspect or plane of the body next to which, in Invertebrates, is the part of the neural 

 axis called ' superoesophageal,' and which, as it supplies nerves to the organs of sense, 

 answers to the brain in Pishes. As in these Vertebrates, also the removal of the neural 

 or dorsal part of the skeleton (PL I. fig. 2) exposes the vascular system (ib. a, a) analogous 

 to the so-called ' aorta ' of Fishes, and homologous with the ' dorsal vessel ' in Insects. 

 In Limnlus the walls of this vasiform heart exhibit muscular and valvular striictures, 

 for the same purpose or office as those of the vertebrate ' heart.' 



In a specimen dissected, with a carapace, or upper crust of the two chief parts of the 

 body, 9 inches in length, the heart was 4 inches 8 lines in length (PL I. fig. 2, «, a). It 

 was included in a delicate membranous sac analogous to a pericardium, but forming, 

 in fact, the wall of a venous sinus. This wall consists of two layers. One may be 

 properly termed a ' tunic :' it includes extremely delicate fibres, chiefly transverse, but 

 reticularly interwoven in a fine cellular bed, the inner surface of which has been the seat 

 of formifaction, or vital crystallization, of particles from the contained fluid, forming an 



t a, fig. 109, ' Lectures on Invertebrata,' 8vo, 1843. 



