XIPHOSURA 457 



In all the others the appendages almost meet in the middle line, but 

 remain distinct. From the posterior surface of the exopodite arise 

 about two hundred branchial leaflets. The appendages are provided 

 with muscles by which the flapping movements are made which propel 

 the animal in a leisurely way through the water and circulate water 

 amongst the leaflets. 



The mouth occupies a subcentral position under the carapace, 

 surrounded by the gnathobases. Worms and small molluscs from 

 the shore mud are seized by the chelae and, after mastication by the 

 gnathobases, stuffed into the mouth, which leads to the fore gut con- 

 sisting of an oesophagus and a chitin-lined "stomach"; the mid gut 

 is long and into it open two pairs of ducts from the digestive glands. 

 These glands are very well developed and fill up much of the space 

 inside the cephalothorax. There are no Malpighian tubules and no 

 salivary glands in Limulus. 



The circulatory system is very complete and like that of the scorpion 

 in its main lines. A unique feature is the complete investment of the 

 ventral nervous system by an arterial vessel which corresponds to the 

 supraneural vessel of the scorpion. 



The nervous system is of a very concentrated type. The supra- 

 oesophageal ganglia supply the eyes and are fused with the ganglia 

 of all the succeeding segments as far as the opercular segment to form 

 a ring round the oesophagus. From this a double ventral cord ex- 

 tends into the opisthosoma, swelling into ganglia in each of the "gill- 

 book" segments. Median and lateral eyes (p. 274) are present. 



The coxal (brick red) glands arise from six segments in the embryo 

 and open on the fifth pair of legs. 



The reproductive organs consist of a network of tubules com- 

 municating with the exterior by paired ducts opening on the genital 

 operculum. The eggs are laid far up on the shore at spring tides in holes 

 dug for them by the mother, and the male, which comes ashore clinging 

 to the carapace of the female, spreads the sperm over them, a method 

 of fertilization very similar to that of the frog. The eggs are heavily 

 yolked and the young hatch as a planktonic larva in a condition re- 

 sembling the adult but with an opisthosoma showing separate segments 

 and without the caudal spine. The larva, which swims by means of the 

 abdominal appendages, as in the adult, has been called the "Trilo- 

 bite" stage, because of an extremely superficial likeness to that group. 



While Limulus has existed since the Trias without any modification, 

 it is of considerable interest that in the Palaeozoic very similar 

 animals occur, in which there are three additional segments and a 

 rather shorter caudal spine, indicating that the latter organ has been 

 formed at the expense of the posterior opisthosomatic segments. 

 These animals are Hemiaspis (Fig. 334 C) and Bunodes. 



