314 THE INVERTEBRATA 



The alimentary canal of the Arthropoda possesses at its mouth and 

 anus involutions of ectoderm, lined by cuticle, which are respec- 

 tively the stomodaeum or fore gut, and proctodaeum or hind gut. 

 These may be short, but in the higher Crustacea and Insecta form a 

 considerable part, and sometimes nearly the whole, of the canal. The 

 cuticular lining of fore and hind gut is shed at moulting. The lining 

 of the fore gut sometimes provides teeth for triturating or bristles for 

 straining the food. Digestion is extracellular, save in certain acarina. 



The respiratio7i of aquatic arthropods, other than those which are 

 but little modified from terrestrial ancestors, is sometimes, if the 

 animal be small, efi'ected only through the general integument of the 

 body, but usually takes place by means oi gills (branchiae). These are 

 nearly always external processes, known as epipodites, which stand on 

 the bases of the limbs, and are often branched or folded. Among 

 terrestrial arthropods, some of the Arachnida possess luftg books, 

 which are generally held to have arisen by the enclosure of gill books, 

 such as those on the limbs of Limulus, each within a cavity of the 

 ventral side of the body. The remainder of the terrestrial Arthropoda 

 breathe by means of tracheae, which are tubular involutions of the 

 ectoderm and cuticle which convey air to the tissues. In some 

 arachnids tracheae are present as well as lung books. Usually tracheae 

 are branched, and strengthened by a spiral thickening of their 

 chitinous lining. The study of the phylogeny of the Arthropoda leads 

 to the conclusion that a tracheal system has arisen independently in 

 the Onychophora, the Arachnida, and the Insecta and Myriapoda. 

 Among the Crustacea, tufts of tubes which resemble tracheae are 

 found in the abdominal appendages of woodlice. 



The vascular system is an '* open " one. That is, be the arteries long 

 or short, they end by discharging their blood not into capillaries in 

 the tissues from which veins conduct it to the heart, but into peri- 

 visceral cavities, known as sinuses, which bathe various organs. From 

 these sinuses the blood collects into a pericardial sinus ("pericar- 

 dium"), part of the haemocoelic system, which surrounds the heart. 

 The latter is a longitudinal dorsal vessel, perforated by ostia by which 

 it receives its blood from the pericardial sinus. Among the conse- 

 quences of the structure of the vascular system are a low blood pres- 

 sure and liability to severe bleeding from wounds. The latter danger 

 is met, especially in the Crustacea, by very rapid clotting of the blood. 

 Haemoglobin is present in the plasma of certain of the lower crustace- 

 ans and a few insects, haemocyanin in Limulus, scorpions, and some 

 spiders. 



The coelom appears in the embryo as the cavities of a series of 

 mesoderm segments (" mesoblastic somites", Fig. 352). It never 

 assumes a perivisceral function, and in the adult is represented 



