135 



jxm in size. The gastrula is flattened somewhat on the 3rd day and in the 

 middle of the ventral surface of the larva an investigation appears that comes 

 into contact with the bottom of the archenteron. At the site of this contact, 

 a secondary mouth forms. In front of the mouth on the ventral side, an 

 elevation forms, the preoral plate. The intestinal tract initially looks like a 

 thin tubule, but later differentiates into sections. The blastopore, which be- 

 comes the anal opening, shifts to the ventral surface. At this stage the larva 

 is called a dipleurula (Figure 91). It begins to feed. The dipleurula stage is 

 very brief 



Figure 91: Asterias amurensis. 

 Dipleurula; coelom rudiments visible. 



Figure 90 : Brachiolaria of Patiria 

 pectinifera. 



Bipinnaria 



The ciliated epithelium of the larva forms two ciliated bands. One of 

 them, the preoral, borders the preoral plate. The second or postoral band 

 proceeds posteriorly along both sides of the larva, passes onto the dorsal side, 

 and extends anteriorly to terminate at the tip of the larval body. The cilia on 

 the rest of the body surface either disappear or are very sparse. However, two 



other ciliated bands form, which are intimately 

 associated with feeding. A preoral depression forms 

 in the mouth region. The peristomal ciliated ring 

 passes along the margin of the mouth, thereby 

 separating it from the preoral depression. The 

 adoral ciliated band originates ft"om the peristomal 

 ring, which passes with two branches deep into the 

 esophagus and forms a loop there. The preoral 

 depression extends laterally along both sides of 

 Figure 92: Asterias amurensis. the larva, bordered by ciliated bands. Thus the 

 Early bipinnaria. larval digestive apparatus is formed (Fieure 92). 



