532 



INVERTEBRATE MORPHOLOGY. 



the other organs for the most part in conformity with the 

 radiation, the arms may be regarded as representing -the 

 radial axes of the body, the interradial axes lying in the in- 

 tervals between them. If now the aboral surface of the disk 

 be examined, there will be found upon it, in one of the inter- 

 radii, a peculiar tubercle, known as the madreporiform tuber- 



?/Vf ■■ 



Bit 



Fig. 346. — Asteriaa arenicola (after AoAssizfrom Vekrill). 



cle, which serves to place the hydrocoel system of canals in 

 communication with the exterior water. There is but one 

 such tubercle, and but one canal leading down from it to the 

 hydrocoel ring which surrounds the mouth, and consequently 

 there can be but one plane in which the animal can be di- 

 vided into two similar parts. Therefore the Starfish, though 

 superficially appearing to possess a radial symmetry, is funda- 

 mentally bilateral — a statement which applies equally well to 

 any member of the Eehinoderm type. 



It does not necessarily follow, however, that the plane 

 which passes through the madreporiform tubercle is the 

 median plane of the body. The larvae of the Echinoderms 

 are strictly bilateral organisms, no sign of radiality being 

 found in them in an early stage of development, and it would 

 seem more satisfactory to take as the median plane of the 



