An Introduction to the Invertebrates, Second Edition - tiera.ru

160 ARTHROPODA: GENERALdifferences: unlike many crustaceans, insects (except mayflies)never moult as adults but have a ‘juvenile hormone’ produced ina separate group of cells (the ‘corpus allatum’). It diffuses tothe epidermis and, at every moult except the last, promotes theappearance of larval form.12.3 How are arthropod internal cavities organised?The organisation of the soft parts of an arthropod is also largelydictated by the cuticle: for example, the absence of cilia is not surprising,since they could not work on the outside and are notneeded on the inside. Cilia prevent cell division and thereforemay direct or constrain development, a function redundant undera cuticle.12.3.1 The haemocoelWith no requirement for a hydrostatic skeleton, the coelom isgreatly reduced. It is represented only in the cavity of the gonadsand, in some aquatic arthropods, in that of the excretory organs.The main body cavity is the haemocoel, divided into blood-filledspaces that bathe all the tissues (Figure 12.5). The blood is movedby muscular contraction. It enters the dorsally placed heart throughthe ostia and is pumped forward into vessels opening into thehaemocoel; the heart beat is under nervous control in arthropods.This open system allows blood to be moved in bulk, which isimportant at moulting, providing turgor and maintaining theshape of the animal immediately after ecdysis. As well as dissolvedfood, hormones and other substances, the blood contains manycells, which may be stationary on surfaces or actively moving, suchas the lurking phagocytes that constitute the immune system.In insects, where there is no separate digestive gland, other cells inthe blood may store and process food.Fig.12.5Diagram of the open blood system in a generalised arthropod.