580 INVERTEBRATE MORPHOLOGY. 



internally by immature germ-cells and is connected at its oral 

 extremity with the ovoid gland. It passes thence to the 

 aboral surface of the body, where it forms a ring (Fig. 266, gr) 

 from which in each iuterradius a branch passes outwards to 

 expand into a highly racemose sac, the reproductive organ 

 (G). In some forms the number of the organs may be re- 

 duced to four or even to two, though five is to be regarded as 

 the typical number. Each organ opens to the exterior by a 

 special duct (Gd), usually opening on a genital plate, but 

 sometimes in an interradius outside the genital plates. 



As already noted, there is considerable variety in the rela- 

 tive positions occupied by the mouth and anus, and many 

 differences of structure are associated with these variations. 

 It is possible, in fact, to divide the Echinoidea into three 

 orders, which are marked out by the positions of the openings 

 of the digestive tract. 



1. Order Desmosticha. 



In these forms the mouth occupies the centre of the oral 

 surface, and the anus approximately that of the aboral sur- 

 face, the radial symmetry usual among Echiuoderms being 

 well marked. The body is usually more or less spherical in 

 form, though occasionally somewhat flattened ; all the arnbu- 

 lacral plates are perforated for the emission of tube-feet, and 

 all five ambulacral areas are equally developed (Fig. 262). In 

 the members of this order, consequently, the bilaterality is 

 marked externally only by the position of the madreporiform 

 tubercle. 



The primary ambulacral plates frequently fuse to form 

 secondary plates each of which is perforated by several pairs 

 of pores, as many as six occurring on some plates in Strongy- 

 locentrotus. The spines are sometimes exceedingly long, as 

 in Diadema, and are usually well developed, being in Arbatia 

 equal in length to about half the diameter of the bod}^. The 

 auriculae are the only representatives of the calcareous plates 

 or bars which extend from the oral to the aboral surface, and 

 an Aristotle's lantern is always well developed, its alveoli 

 being much longer than broad. In this order external branchiae 



