8o A Guide to the Zoological Collections in the 



the King-crab — has long been included among the Crusta' 

 ceUy but the researches of modern embryologists show 

 that it has much nearer affinities with the next class of the 

 Arthropoda, namely, with the Arachnida or Spiders, 



The body of Limulus, which is enclosed in a chitinous 

 armour, consists of three portions — (i) a large horse- 

 shoe-shaped cephalothorax, (2) a hexagonal abdomen, and 

 (3) a long rigid post-anal spine. The cephalothorax 

 bears on its convex upper surface at the sides two large 

 compound eyes, and in front two small simple eyes ; and 

 in its concavity are seen six pairs of legs, the basal joints 

 of which are thickly beset with spiny teeth and serve as 

 jaws. The legs of Limulus are, in short, in what is known 

 as an indifferent condition, being used both in progression 

 and prehension, and in the mastication of food, and they 

 illustrate excellently the fact that the jaws and legs of 

 Arthropoda are "homologous organs," in other words that 

 Arthropod jaws are derived from legs. On the under sur- 

 face of the abdomen are five pairs of broad leaf-like feet, 

 all of which carry a series of gill-plates arranged like the 

 leaves of a book. 



Limulus \s ionxid in India in the muddy salt waters of 

 estuaries. 



APPENDIX TO CRUSTACEA No. II. 

 3. ARTHROPODA PYCNOGONIDA. 



[Western SEiiU-casc 58]. 



Pycnogonum — the " Sea-spider ^' — has been at different 

 times classed with the Crustacea ^x^^ with the Arachnida : 

 it differs from both, and by modern zoologists is placed in 

 a class by itself. 



pycnogonum has a long suctorial proboscis with a pair 

 of many-jointed palps at its base, and a very small abdo- 

 men with four pairs of long many-jointed legs. The male 

 has in addition a pair of shorter legs, which spring from 

 the back, and are used for carrying the eggs during the 

 development of the young. 



