VASCULAR SYSTEM OF AETHROPODA. 279 



The luminous orgaus of the Lampyridfe are special modifica- 

 tions of the fat-body. They are formed of plates of cells, to which 

 a large number of tracheal and nerve branches are sent. On their 

 inner side they are covered by other cells, which are not luminous, 

 but which are loaded with a large number of urinary concretions. 

 The superficial position of the luminous plates shows that they 

 belong to the epidermal layer (liypodcrm) . 



The regularity of the coelom, along the long axis of the body, is 

 modified by the muscular system. Where this is greatly developed 

 (as in the cephalothorax of the Crustacea and Arachnida, and in 

 the thoracic metameres of the Insecta) only a small space is left for 

 the coelom. The processes, too, of the chitinous skeleton are the 

 cause of variations in it ; chiefly by forming smaller cavities, as they 

 especially do in the Crustacea. In the Insecta a subneural cavity is 

 formed by muscles, which in some of them are inserted into the 

 chain of ventral ganglia. In others similar muscles run horizontally 

 from one side to the other of the abdomen^ and so likewise mark off 

 a portion of the coelom. 



Vascular System. 

 § 216. 



This system of organs, which in the Vermes had become liighly 

 developed, seems to be less so in many of the Arthropoda, chiefly 

 because the coelom generally forms a portion of the vascular system. 

 There is, therefore, no difference between the blood and a peri- 

 enteric fluid. 



As a rule, a dorsally-placed vascular trunk is alone de- 

 veloped to any great extent ; this functions as a heart, and seems to 

 be homologous with the dorsal vascular trunk of the Vermes, parts 

 of which may also function as hearts. But it seems to differ by not 

 being connected with the enteron. The blood is either driven to 

 the anterior, or to both ends of the body, by the cardiac tube. This 

 dorsal cardiac tube of the Arthropoda is not, however, provided 

 with afferent vessels, and the blood which passes into it does so by 

 narrow venous ostia; so that, although a peripheral system of 

 vessels may in some divisions be formed from continuations or rami- 

 fications of the arterial vessels, or by the differentiation of vascular 

 canals from portions of the coelom, yet close to the heart a sinus is 

 formed from a portion of this latter. This "pericardial sinus" 

 appears, therefore, to be a portion of the coelom ; and the sbght 

 development of the vessels which obtains in many Arthropoda is not 

 to be regarded as a degeneration from a more perfect stage, but as 

 a low stage, which is correlated with a less amount of development. 

 It is still an open question whether, and how, this simpler form of 

 the vascular system is connected with the arrangements which are 

 found in Vermes. 



