THE MEROSTOMATA. 231 



The heart, in Limulus polypJtemus, is an elongated mus- 

 cular tube, divided into eight chambers, and having as many 

 pairs of lateral valvular apertures. It lies in a large peri- 

 cardial sinus, which, in its abdominal portion, presents on 

 each side five apertures, the terminations of the branchial 

 veins. The branchia3 consist of numerous delicate semicir- 

 cular lamelhc, attached transversely to the posterior faces of 

 the five post-opercular appendages, and superimposed upon 

 one another like the leaves of a book. 



The nervous system appears, at first sight, to be very con- 

 centrated, its principal substance being disposed in a ring, 

 embracing the oesophagus ; but, on closer inspection, it is 

 found to consist of an anterior mass, representing the prin- 

 cipal part of the cerebral ganglia in most other Crustacea^ 

 and of two ganglionic cords which proceed from the outer 

 and posterior angles of that mass, and extend as far as the 

 interval between the last and penultimate pairs of append- 

 ages. These cords are thick, and lie on each side of the 

 aesophagus, around which they converge, so as to come into 

 close union and almost confluence, immediately behind it. 

 In front of this point, however, they are connected by three 

 or four transverse commissures, which curve round the poste- 

 rior wall of the oesophagus, and become gradually shorter 

 from before backward. 



Tiie first of these commissures unites the two cords oppo- 

 site the origin of the nerves to the third pair of appendages, 

 which I rejrard as the homologues of the mandibles. In front 

 of this point, the cerebral ganglia give off, from their ante- 

 rior edges, the nerves to the ocelli, eyes, and frontal region ; 

 and, from their posterior and under surfaces, those to the an- 

 tennules. The nerves to the antennae arise from the cord 

 close to the outer and posterior angles of the cerebral gan- 

 glia, and some distance in front of those to the mandibles. 

 Close behind the latter arise the large nerves to the fifth and 

 sixth cephalo-thoracic appendages. 



The nerves to the rudimentary seventh pair of append- 

 ages are slender, and arise rather from the under part of 

 the post-oesophageal ganglia; those which supply the eighth 

 pair of appendages, constituting the operculum, are also 

 slender, and seem to come off from the two longitudinal com- 

 missural cords, which connect the post-oesoph.'igoal ganglia 

 with those which are situated in the second division of the 

 body, though they are, in truth, only united in one sheath 

 with them for a short distance, and can be readily traced to 



