122 ESSENTIALS OF ZOOLOGY 



latter is continued through the basal membrane as the nerve 

 (see p. 106). 



Excretory Organs. The excretory organs are the Malpighian 

 tubules, which in the form of a large number of fine tubules 

 encircle the anterior end of the proctodeum, and project 

 into the haemocoel surrounding the gut. They take up the 

 excretory products gathered by the blood, and the excretion 

 is discharged through the intestine. The fat body also takes 

 part in excretion (see below). 



Body Cavity. It has been plain from the foregoing account 

 that the cavity of the body is a primary body cavity and 

 almost completely a haemocoel. The blood bathes all parts 

 of the space between skin and alimentary canal, and this space 

 is crossed by the numerous tubes of the respiratory organ, 

 and the walls of the cavity are occupied by nerves which, like 

 the tracheae, have a universal distribution in the body and 

 appendages. The haemocoel contains a loose connective tissue, 

 especially around the abdominal viscera, and its cells are 

 distended by fat. This is the tissue called the fat body, and 

 appears to be derived from the coelom mesoderm. It is 

 largely developed in insects generally, and is very conspicuous. 

 It is regarded as a reserve of food material, and appears also 

 to take a share in excretion. The coelom is therefore prac- 

 tically non-existent. It is represented merely by the gonocoel 

 in each sex, and the ducts of these are coelomiducts. 



Reproductive Organs. Male. There is a pair of testes, 

 difficult to isolate from the fat body. Each testis opens into 

 a fine vas deferens, and the two vasa open into the mushroom- 

 shaped seminal vesicle. From the seminal vesicle the ejacu- 

 latory duct leads to the external aperture below the anus. 

 The aperture is surrounded by plates and processes, called 

 gonopophyses, and with these are associated the styles. 



Female. The ovaries consist on each side of eight tubes 

 in which the developing ova are plainly visible. The tubes 

 unite posteriorly to form the oviduct, and the two oviducts 

 unite to open by a single vertical aperture to the exterior. 

 Between the opening and the anus are strong chitinous pro- 

 cesses, the gonopophyses. Accessory to the female organs are 

 the spermathecae, two sacs which open on the ninth segment, 



