ARTHROPODA 165 



Natural History 



Class I. Crustacea. — Crustacea vary in size from microscopic to 

 the 34-pound lobster. Nearly all have a hard exoskeleton. They 

 shed this and grow for a time. Young crayfishes and lobsters 

 moult 8 times the first year, 5 times the second year, 3 times the 

 third year and from one to three times annually, thereafter. They 

 are divided into the head, thorax and abdomen. The head and 

 thorax are frequently fused to form a cephalothorax. There are 

 usually 21 segments in higher Crustacea. Seven is the typical 

 number of abdominal segments. There are eight in the cephalo- 

 thorax (5 legs and 3 appendages) and six in the head. 



Subclass Malacostraca. Order Decapoda. Type — Lobster or 

 Crayfish. (Figure 73.) Digestive System. — This consists of the 

 esophagus, cardiac stomach, pyloric stomach, intestine, and liver. 

 The esophagus is lined by chitinous cuticle and supplied with small 

 " salivary " glands. The cardiac stomach contains the gastric mill, 

 which is equipped with powerful chitinous teeth, the most important 

 being the two laterals and the single dorsal. The posterior pyloric 

 stomach has a sieve-like strainer of coarse hairs. The short mid-gut 

 receives the secretions from the lateral liver or hepato-pancreas. 

 This gland absorbs peptones and sugar, manufactures glycogen and 

 furnishes enzymes corresponding to the gastric and pancreatic 

 juices of mammals. The intestine is long and straight, except for a 

 slight dilatation in the sixth abdominal segment, the rectum. An 

 intestinal cecum is dorsally situated. Chitin-lined, the intestine 

 has but few glands. Both mouth and anal opening are ventrally 

 situated. 



Circulatory System. — The blood is a colorless liquid with amebo- 

 cytes. In Crustacea., hemoglobin is replaced by a fluid containing 

 copper instead of iron, which is called haemocyanin. It is colorless 

 when flowing, but turns plumbago colored when exposed to the air. 

 The heart is shield shaped with three pairs of ostia. There are seven 

 arteries and many sinuses. The ophthalmic artery supplies the 

 esophagus, stomach and head. Two antennary arteries supply the 

 stomach, antennae and excretory organs. Two hepatic arteries 

 supply the digestive glands. There are also dorsal abdo?ninal arteries, 

 the sternal artery which passes through the spinal cord, the ventral 

 thoracic, and the ventral abdominal. 



Circulation of the Blood.— The heart sends the blood to the 

 arteries, capillaries, sternal sinus, and to the^///j where it is purified. 



