ECHINODERMA 5 



stored up in special places in the body or wander out through 

 the body wall and are thus destroyed. These cells are formed 

 within a glandular organ that accompanies the stone canal, the 

 axial organ, or dorsal organ. It was formerly, wrongly, regarded 

 as a heart. 



Regarding the arrangement of the genital organs, reference 

 must be made to the sejiarate classes. Most echinoderms have 

 separate sexes, but some forms are hermai^hrodites. There is 

 usually no external difference between the two sexes. External 

 sexual organs are not found, and no copulation takes place. The 

 sexual products are discharged into the water, Avhere eggs and 

 sperm meet. In those forms which are viviparous internal fer- 

 tilisation must take place ; in such cases the spermatozoa must 

 find the eggs in the place where development is to take place 

 (the body cavity or elsewhere). 



The organs of digestion consist in the sea-stars and brittle-stars 

 of a large sac-shaped stomach ; in the other classes of a long 

 sinuate intestinal canal attached to the body Avail by means of a 

 mesentery. All brittle-stars and some sea-stars lack an anal 

 opening, and, accordingly, indigestible matter is discharged 

 through the mouth. In sea-stars a pair of large folded caeca 

 proceed from the stomach into each arm, hanging in a mesentery, 

 and form the liver. 



The echinoderms pass through a remarkable and very com- 

 plicated metamorphosis. The larvae are free-swimming (pelagic), 

 and so totally different from the adult animals in structure 

 and appearance that at first they were taken to be separate 

 animals, being described under special names — ■ Pluteus, 

 Bipinnaria, Brachiolaria, Auricularia — names which are still 

 partly retained. 



The larvae are especially characterised by their vibratile 

 or ciliated band ; this band, which is closely set with micro- 

 scopical hairs (cilia), forms the locomotor organ of the larva. 

 At first it is a simple band surrounding the mouth, but 

 graduall}^, as the larva grows, it forms folds or armlike out- 

 growths (processes) ; these are of constant occurrence and may 

 be homologised in the various larval types. The names of 

 the various j^rocesses or arms are seen in Fig. 3. 



The four main types of pelagic echinoderm larvae : Auri- 

 cularia (Holothurian larvae), Bipinnaria (Asteroid larvae) ^ 



^ The name Brachiolaria is sometimes used for a group of Asteroid 

 larvae which are, in a later stage, provided with special papillate arms and 

 a sucking disk. 



