STRUCTURE OF INSECTS. 267 



cavity, known as the pericardium, the heart lies, swayed by 

 special muscles. It is a long tube, usually confined to the 

 abdomen, usually of eight chambers, with paired valvular 

 openings on its sides, through which blood enters from the 

 pericardium. The blood is driven forwards, the posterior 

 end of the heart being closed, and there is usually an anterior 

 aorta or main blood-vessel. But for the most part the blood 

 circulates in spaces within what is commonly called the body- 

 cavity. Such a circulation is often described as lacunar. 

 The blood may be colourless, yellow, red, or even greenish, 

 and in some cases haemoglobin, the characteristic blood- 

 pigment of Vertebrates, has been detected. The cells of 

 the blood are amoeboid. 



Body-Cavity. — One is apt to use this term in two senses — 

 for the primitive body-cavity or coelome, and for the apparent 

 body-cavity of the adult. In discussing the development of 

 Peripatus, Sedgwick notes the following characteristics of a 

 true coelome : — It is a cavity which (i) does not communicate 

 with the vascular system, (2) does communicate by nephridial 

 pores with the exterior, (3) has the reproductive elements 

 developed on its lining, (4) develops either as one or more 

 diverticula from the primitive enteron (or gut), or as a space or 

 spaces in the unsegmented or segmented mesodermic seg- 

 ments. Now, in Arthropods the apparent body-cavity is not a 

 true coelome, it consists of a set of secondarily derived vascular 

 spaces ; it has been called a pseudocoele, or a haemocoele. 

 The true ccelome of Arthropods is very much restricted in 

 the adult, all the more so that most Arthropods (e.g.. Insects) 

 have no distinct nephridia. 



But the apparent body-cavity in which the organs lie, and 

 in which the blood circulates, is well-developed in Insects. It 

 includes, inter alia, a peculiar fatty tissue, which seems to be 

 a store of reserve material, which is especially large in young 

 insects before metamorphosis, and is also interesting as one 

 of the seats of "phosphorescence" in those insects which 

 glow. 



Excretory System. — Although no structures certainly homo- 

 logous with nephridia have yet been demonstrated in 

 Insects, the excretory system is well-developed. From the 

 hind-gut (proctodaeum), and therefore of ectodermic origin, 

 arise fine tubes, or in some cases solid threads, which extend 



