268 ARACHNIUA XIPHOSURA CHAP. 



walking legs, and these by an alternate motion " card " the food 

 into fragments, which when sufficiently comminuted pass into the 

 mouth. At times its appendages are caught between the valves 

 of Venus meixenaria, a burrowing bivalve known in America as 

 the " quahog " or " round clam." The Limuhts has seized with 

 its chelate claws the protruding siphon of this mollusc, which, 

 being rapidly drawn in, drags with it the limb of the king-crab, 

 and the valves of the clam are swiftly snapped to. 



As a rule in Arachnids the alimentary canal is no longer 

 than the body, and runs straight from mouth to anus, but in 

 Limulus, the mouth being pushed far backward, there is a median 

 loop, and the narrow oesophagus which leads from the mouth, 

 having traversed the nerve-ring, passes forward towards the 

 anterior end of the carapace. Here it enters into a somewhat 

 Z) shaped and spacious proventriculus ; posteriorly tlie proventri- 

 culus opens by a funnel-shaped valve into the anterior end of the 

 narrow intestine. All these structures are derived from the 

 stomodaeum, are lined with chitin and are provided with very 

 muscular walls whose internal surface is throwir into longitudinal 

 ridges. The intestine runs straight backward, diminishing in its 

 diameter, and ends in a short, chitin-lined, and muscular rectum 

 which is derived from the proctodaeum ; the anus is a longitudinal 

 slit. A large gland, usually called the liver, consisting of in- 

 numerable tubules, pours its secretions into the broader anterior 

 end of the intestine by two ducts upon each side ; it extends 

 into the meso- and meta-soma, and, together with the repro- 

 ductive organs, forms a " packing " in which the other organs are 

 embedded. The contents of the alimentary canal are described as 

 " pulpy and scanty," and probably much of the actual digestion 

 goes on inside the lumen of the above-mentioned gland. 



The vascular system of Limulus, like that of the Scorpions, is 

 more completely developed than is usually the case in Arthro- 

 pods. For the most part the blood runs in definite arteries, and 

 when it passes as it does into venous lacunae these are more 

 definite in position and in their retaining walls than in other 

 members of the phylum. 



The heart lies in a pericardial space with which it communi- 

 cates by eight ^ pairs of ostia. Eight paired bands of connective 

 tissue, the " alary muscles " of authors, sling the heart to the 



' A rudimentary ninth pair of ostia are described anteriorly. 



