84 



NATURAL HISTORY OF ARTHROPODS. 



claw, there being a difference in the different species and sexes. The sixth pair, how- 

 ever, terminates in two Hat spines, and just above these on either foot is a whorl of 

 similar plates. These are of great use to the animal in walking, as they give a firm 

 foothold in the soft mud and sand. Between the bases of the sixth pairs of feet is the 

 metastoma, a pair of single-jointed appendages. 



The abdomen above shows indistinct traces of its original segments, while its 

 lateral margins are armed with movable spines. Beneath are found the gills. These 

 are composed of numbers of thin plates, arranged like the leaves of a book, and sup- 

 ported on five pairs of greatly modified limbs, the whole being covered by the anterior 

 pair of abdominal appendages which form the operculum. 



FIG. 115. Longitudinal section of Limulus. a. Anus. b. Brain, c. Cone. cp. Cartilaginous plate. /(.Heart. 

 i. Intestine. I. Liver, m. Mouth. . Nervous cord. o. OEsophageal ring. p. Proventriculus. s. Stomach. 



From the mouth the alimentary canal first goes forward, widening out into a crop 

 or proventriculus, the Avails of which are lined with chitinous folds. This crop bends 

 on itself and communicates by a small tube with the intestine, which pursues a straight 

 course to the end of the abdomen. The liver and genital organs occupy the greater 

 part of the body. The heart is an elongate organ, and the nervous system consists of 

 a ring around the oesophagus, from which arise the nerves supplying the cephalo- 

 thoracic members, and a long cord which gives off nerves to the gills, etc. One 



remarkable peculiarity should be mentioned here: The oeso- 

 phageal ring and the nervous cord are ensheathed in the ven- 

 tral arteries so that they are bathed with the blood. 



It is extremely probable that the same individuals deposit 

 their eggs more than once each year, the favorite time being 

 at the time of the full moon, when the tides run unusually high. 

 The eggs and milt are placed in hollows near high-water mark, 

 and are covered with sand by the retreating tide. The time 

 of spawning is in the months of May, June, July, August, and 

 probably in the Southern States, March and April. The eggs 

 are small, resembling in appearance seed-pearls, and are not 

 easily distinguished from the sand in which they are deposited. 

 Of the early stages of development nothing certain is yet known. By some process 

 the eggs become enveloped at an early day with a cellular membrane, and subsequently 

 the six pairs of cephalothoracic limbs appear simultaneously. Next, the body segments 



FIG. 1 16. Early stage of 

 Limulus embryo. 



