TYPE ECHINODERMA. 533 



adult animal one which corresponds as closely as possible 

 with the larval median plane. The madreporiform tubercle, 

 or rather the pore which corresponds to it, and the tube 

 which leads from it to the rudiment of the hydroccel system 

 can readily be made out in the larvae of most forms, and it 

 can be seen that it lies to the left of the median plane of the 

 body. Indeed in the larvae of some Starfishes two pores 

 occur at an early stage of development, one to the left and 

 the other to the right of the median plane, the latter subse- 

 quently disappearing. The madreporiform tubercle might 

 therefore be regarded as lying to the left of the median plane, 

 which w r ill accordingly pass through the radius to the right 

 of the tubercle. 



However, it is impossible to tell how much modification 

 has taken place during the transition from the bilateral to 

 the radial condition, and it is not impossible that the greater 

 portion of the adult represents one of the halves of the 

 embryo, the other half remaining more or less undeveloped. 

 Furthermore a secondary bilaterality supervenes in certain 

 forms of Echinoidea and Holothuroidea which does not agree 

 with that indicated in the preceding paragraph, and is not 

 indeed the same in the two groups. It seems therefore pref- 

 erable to assume a perfectly arbitrary method of indicating 

 the radii of the body, calling that radius which lies opposite 

 the madreporiform tubercle A, that which lies to the left of 

 this when the animal is held with the oral surface upwards 

 B y and so on C, D, E, following the direction of the hands of 

 a watch. The interradii may be indicated by combining the 

 letters of adjacent radii, the interradius between A and B 

 being denoted by AB. 



The body-wall in the Echinoderma is covered on the out- 

 side by a usually delicate, and in some cases ciliated, ecto- 

 derm, which may, however, be indistinguishable from the sub- 

 jacent mesodermal tissues in certain parts of the body. 

 Below this ectoderm, when present, comes a layer of meso- 

 dermal connective tissue consisting of relatively few cells 

 imbedded in a more or less fibrillar matrix, and in this con- 

 nective tissue there are imbedded numerous calcareous plates, 

 in some forms, such as the Holothuriaus, somewhat widely 



