TYPE ECH1NODERMA. 583 



somewhat heart-shaped, owing to the anterior radius D being 

 more or less depressed so as to form a groove (Fig. 263). 



The ruadreporiforrn tubercle extends usually through the 

 centre of the apical system into the posterior iuterradius, 

 thus dividing the apical system. The posterior genital pore 

 is obliterated in all members of the order, and the reproduc- 

 tive gland corresponding to it disappears, so that but four 

 reproductive organs and pores are present (Spatangus). In 

 some forms, however, the reduction of the reproductive organs 

 and pores is carried still further by their disappearance in the 

 right anterior iuterradius CD, and those of the left anterior 

 iuterradius DE may also disappear, the genital plate becom- 

 ing part of the madreporiforrn tubercle, so that the number of 

 reproductive organs and genital pores may be reduced to two 

 (Moira). The ambulacra are usually dissimilar, especially in 

 heart-shaped forms, in which the anterior ambulacrum be- 

 comes much modified. Fascicles are generally present, ar- 

 ranged in different manners in different species, in some sur- 

 rounding the ambulacra, in others forming a ring in the vicinity 

 of the anus, and in others arranged in numerous patches or 

 lines. Spines are abundantly present and are usually of a 

 moderate length. The mouth is bounded behind by a well- 

 marked lip or labrum, produced by the extension forwards 

 below the mouth-opening of the plates of the posterior inter- 

 ambulacruni AB. There is no Aristotle's lantern in the 

 Petalosticha. 



Development of the Echinoidea. The development is in its general 

 features very similar to that of the Ophiuroids, the free-swimming larvae 

 having a closely-similar form and being known by the same name. The 

 Eehiuoid Pluteus (Fig. 261) may, however, in some cases be distinguished 

 by the occurrence of two (Arbacid) or three (Spatangus) processes upon 

 the posterior portion of the body, which are wanting in the Ophiurid larvae; 

 and furthermore in some Echinoid Plutei (Arbacid) two earlike lobes fringed 

 with cilia occur upon the sides of the posterior portion of the body and 

 are known as ciliated epaulettes. The young Sea-urchin develops in the 

 posterior portion of the body of the larva as in other forms, the armlike 

 processes and the praeoral lobe of the larval body being gradually resorbed. 



The relationships of the various groups have already been referred to 

 and need not be again discussed here, except to repeat the statement that 

 the evidence at our disposal indicates that the Desuiosticha are the most 



