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 ⸧ shaped and spacious proventriculus; posteriorly the proventriculus 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 thrown 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 innumerable tubules, pours its secretions into the broader anterior end of the intestine by two ducts upon each side; it extends into the meso- and metasoma, and, together with the reproductive 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 Arthropods. 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 communicates by eight[216] pairs of ostia. Eight paired bands of connective tissue, the “alary muscles” of authors, sling the heart to the pericardial membrane. Posteriorly the pericardial chamber receives five paired veins on each side coming from the gills and returning the purified blood to the heart.