STRUCTURE OF LIMULUS. 295 



The Nervous System. — The supra-oesophageal brain gives off nerves 

 to the eyes. United to the brain are two ganglionated and transversely 

 connected cords forming an oval in the cephalothorax, giving off nerves 

 to the limbs, and continued into a ganglionated abdominal cord. 



There are two ' ' compound " eyes lying towards the sides of the 

 cephalothoracic shield, and in front of these two more median simple eyes. 

 The compound eyes are covered by a layer of chitin continuous with 

 that of the shield, and the various eye-elements are so remarkably 

 distinct from one another, that the eye might be called a group of 

 simple eyes. 



The Food- Canal. — Worms and other animals seized by some of the 

 pincers, are partly masticated by the bases of the five posterior cephalo- 

 thoracic legs. From the mouth the fore-gut bends upwards and forwards 

 into a crop with internally folded chltinous walls. The mid-gut extends 

 along the cephalothorax and abdomen, and in the former bears two 

 pairs of large hepato-pancreatic digestive outgrowths. The hind-gut is 

 short and ends in front of the base of the spine. 



Two reddish coxal glands lie in the cephalothorax, and open in young 

 forms at the bases of the fifth appendages. 



Tile Vascular System. — The heart lies within a pericardium and is 

 eight -chambered, with eight pairs of valved ostia. Hiemocyanin is 

 present as usual as the respiratory pigment of the blood. From an 

 anterior aorta like that of the scorpion, two vessels are given off which 

 bend backward ventrally, ensheathing the oesophageal nerve-collar and 

 its continuations. There are also four pairs of lateral vessels from the 

 heart which unite in a longitudinal trunk on each side. From capil- 

 laries, the blood is gathered into a ventral venous sinus, whence it passes 

 to the respiratory organs, and thence to the pericardium and heart. 



The respiratory organs are borne by the last five appendages. They 

 look like much plaited gills. Their leaf-like folds are externally washed 

 by the water, within them the blood flows. According to Ray Lan- 

 kester, they are comparable structurally to the pulmonary sacs of the 

 scorpion. 



The Reproductive System. — The males are smaller than the females. 

 The testes are very diffuse, the two vasa deferentia open on the internal 

 surface of the operculum, and the spermatozoa, which are vibratile, seem 

 to be shed into the water. The ovaries form two much branched but 

 connected sacs ; the oviducts are separate, and enlarge before they open 

 beneath the operculum. 



Spawning occurs in the spring and summer months, the favourite time 

 being at the full moon, when the tides run unusually high. The ova and 

 spermatozoa are deposited in hollows near high-water mark. Some of 

 the early stages of development, still imperfectly known, present con- 

 siderable resemblance to corresponding stages in the scorpion. In the 

 larvfe, both cephalothorax and abdomen show signs of segmentation, but 

 these disappear. The spine is represented only by a very short plate, 

 and the larva presents a striking superficial resemblance to a Trilobite. 



It seems likely that Limulus is linked to the extinct Eurypterids by 

 some fossil forms known as Hemiaspidse, e.g., Hemiaspis, Belinurus, 

 Bunodes. 



