DIGESTIVE CANAL OF ARTIIEOrODA. 



277 



(Campodea), but four arc present in Lepisma. The function of the 

 urinary canals is notably increased in the Insecta, which have a 

 perfect metamorphosis, during their larval stage, as is shown not 

 only by the great development of these organs (Fig. 139, vm), but 

 also by the quantity of urine which is collected in the hind-gut 

 during their pupal stage. This phenomenon corresponds exactly 

 to that period in which the most intense plastic activity is going on 

 in the organism, in connection with the 

 development of the perfect body. It is 

 clear that the function of the Malpighian 

 canals of the Insecta is not to be found 

 exclusively in the excretion of urine ; but 

 that an older hypothesis, according to which 

 they were regarded as organs for the excre- 

 tion of bile also, is not altogether without 

 justification, is seen from the fact that por- 

 tions of this canal have a different struc- 

 ture, while the secretion in these portions is 

 different also. 



The brownish-yellow, or light-yellow 

 coloration of the urinary canals is due to 

 the substances deposited in the cells of the 

 canal-wall, and is more intense when secre- 

 tion is more active. Most of the Diptera 

 (Fig. 142, B vm) and Hemiptera have two 

 urinary canals, which are connected with 

 one another; there are six canals in the 

 Lepidoptera, in many Nocturnal flies, and 

 in several Pseudoneuroptera (Termites) ; 

 there are four to six in the Coleoptera; 

 the Hymenoptcra are distinguished by a large number of short 

 urinary canals, and indeed hundreds have been found in them, and 

 in many Orthoptera (Fig. 142, A vm). As a rule they are seldom 

 branched; but we often meet with loop-like connections between 

 their ends. They open at apparently very different points, accord- 

 ing to the length of the hind-gut. They open very far forwai'ds in 

 the Cicadas, Diptera, and Lepidoptera. In the Hymenoptera they 

 open just behind the mid-gut. 



Where the canals unite to form a common excretory duct there 

 is a circular layer of muscles (Gryllotalpa). Separate circular fibres 

 are very seldom found in the secretory canals (Brachinus). 



Fig. 144 Portion of a Mal- 

 pighian vessel of Muse a 

 vomitoria. tr Trachea?. 

 I Lumen, ft Nucleus. 



Coelom. 



§215. 



As the embryonic body v,± differentiated, a cavity, the coelom, is 

 formed in the endoderm between the enteron and the wall of the 



