THE CRAYFISH 129 



mid-gut. The caeca are lined by a glandular epithelium, con- 

 taining numerous yellow fat globules, and zymogen granules. 

 These are eventually discharged into the lumen of the caecum, 

 and pass through the ducts into the gut. There are no other 

 glands connected with the gut. 



The circulatory system of the crayfish consists of a muscular 

 heart, a number of efferent tubular vessels or arteries with 

 definite walls, and a system of wide lacunar blood sinuses 

 like those of Apus. 



The heart is median and dorsal, lying in the thorax, beneath 

 that part of the carapace bounded by the branchio-cardiac 

 grooves. It is not an elongated tube like that of Apus, but 

 a short rhomboidal sac with muscular walls, and three pairs 

 of valvular ostia opening into the pericardial sinus in which it 

 lies. One pair of ostia is dorsal, another pair lateral, and the 

 third pair ventral. The heart is connected with the walls of 

 the pericardial space by six bands of fibrous tissue, the alae 

 cordis, similar to the structures called by the same name in 

 Apus. Five arteries spring from the anterior end of the heart. 

 Of these the two outermost are given off from the under side, 

 and pass straight to the digestive glands ; hence they are called 

 the hepatic arteries. Inside, and above the hepatics, an 

 antennary artery passes out from the heart on either side, 

 runs forwards along the sides of the proventriculus, and divides 

 into two branches one, the gastric artery, going to the pro- 

 ventriculus, the other running forward to the head, and giving 

 off branches to the antennae and green gland. Between the 

 antennaries is a median unpaired vessel, the ophthalmic artery, 

 which runs straight forward in the middle line over the surface 

 of the proventriculus, and supplies the anterior part of that 

 organ, the oesophagus and head. Posteriorly, a single median 

 artery is given off from the heart, and it immediately divides 

 into two branches. One of these, the superior abdominal 

 artery, runs back, just above the intestine, to the hinder end 

 of the body, giving off numerous branches to the gut and 

 abdominal muscles. The other branch, the sternal artery, 

 runs vertically downwards, passes between the ganglia, supply- 

 ing the sixth and seventh pairs of thoracic limbs, and divides 

 below the nerve chain into an inferior abdominal artery 

 running backwards, and an inferior thoracic artery running 

 forwards ; these vessels supply the ventral muscles and limbs 



ii. i 



