OF STARFISHES. 185 



The main feature, though, of this fluid circulating' system, is the 

 large canal (m 1 ) which extends from the upper side of the circu- 

 lar tube (aq) and backwards along the superior median line of the 

 body, and terminates in a trumpet-like dilatation (m). On the 

 outside of the body the mouth of the trumpet is covered by a 

 circular, stony button which is peculiarly ornamented by laby- 

 rinthine furrows, that give it the appearance of a minute 

 brain-coral. On this account it has been called the madrepori- 

 form body. It is readily detected, in our common starfishes, as 

 a little, faintly rose-colored, hard, circular plate, (fig. 109, m,) 

 from one eighth to one fifth of an inch in diameter, lying at the 

 angle between two arms. 



Now, in all the numerous and various kinds of Starfishes and 

 Trepangsthe trumpet-shaped body, (m 1 ,) which is called the mad- 

 reporic canal, may be found in some guise or other at the median 

 line of the body ; and serves as a guide by which we may at a 

 glance determine the right and left sides. How closely related 

 in their arrangement the other organs are in reference to this 

 one you will see presently in a most convincing illustration ; it 

 is, therefore, now spoken of in this light, that it may be referred 

 to hereafter as the eminently prominent, material symbol of the 

 bilateral idea, in the midst of such a great frequency of rep- 

 etitions among the various organs which are more or less in- 

 timately connected with it. Through whatever other line you 

 may divide the body than the one in which this is situated, you 

 get an asymmetrical figure.* 



* Lest some misapprehension or objection should arise in the mind of any one 

 who may bring to his recollection the asymmetrical lungs of serpents, or the one- 

 sided reproductive organs of birds, &c., &c., I would state that these are totally 

 different cases. These organs were perfectly symmetrical in the younger stages 

 of growth, and clearly originated upon a basis of bilaterality ; so that any sub- 

 sequent changes in their proportions could not aflect the typical relations of the 

 other organs, any more than would the opening of one eye and the shutting of 

 the other. In the madreporic canal we have a sort of base-line upon which all 

 the other organs are built up, and from which they are, as it were, an ideal out- 

 growth and dependency, subservient and secondary to the influence which it 

 exerts as the ideal of the "primitive stripe." The well-known instances of ec- 



