38 



IV. 



General arrangement of the various organic systems, 



(Topography). 



If the various organs described in the foregoing lines are considered in connexion 

 with each other, we shall find that each arm (antimeron) contains in itself all the most im- 

 portant organic systems (vital organs). In the disc these organic systems are so connected 

 that the whole complex of arms forms a common organism. We have thus in fact in the 

 single arm a complete expression for the whole organisation. The complete star-fish is in 

 other words to be considered as a multiple of arms or rays arranged around a common centre 

 and (in the disc) brought into organic connexion with each other. In the disc we find there- 

 fore chiefly connecting parts (commissures) between the various organic systems of the arms; 

 while the real principal parts of the organs have their place in the arms themselves. To 

 enumerate separately the particular organic systems above noticed: the commissure of the 

 ambulacral skeleton is formed by the so-called oral ring, or, (as the ambulacral vertebra 

 contained in the same are flush with the skeleton of the arm, and in reality represent the 

 interior continuation of the same) more correctly speaking, only by the connecting plates 

 peculiar to the disc viz. the wedge-plates and the parietal plates; the commissure of the 

 cuticular system is formed by the dorsal skin of the disc; the commissure of the water 

 system, -by the circular ambulacral vessel; the commissure of the nervous system by the 

 circular nervous band lying in the periphery of the oral cuticle; the commissure of the 

 digestive system, by the central stomach, with its single oral aperture; the commissure of 

 the blood-system, by the cavity of the disc surrounding the stomach and by the circular 

 blood-sinus contained between the lamellae of the oral cuticle. 



The special organic systems of the disc are only the apparatus of secretion and the 

 so-called ,,heart", if we do not reckon this last as belonging to the blood-system. 



The special organs of the arms are the organs of generation and the terminal 

 organ of sense. 



The foundation of each single arm consists again of a series of similar consecutive 

 joints or metamera, each of which contains its own section of the skeleton, water-vessel, 

 nervous and muscular systems, with a corresponding pair of external members (Water-feet). 



