108 



vesicles that lie along the sides of the intestine — the left and right coeloms. 

 Each coelom later divides into three parts — anterior (axocoel), middle 

 (hydrocoel), and posterior (somatocoel); the left coeloms are better devel- 

 oped. Such is the typical picture. But in some species, the formation of 

 coeloms may be accelerated by independent formation of the left somatocoel 

 (in Patiria miniata — Heath, 1917; Henricia sanguinolenta — Masterman, 

 1902; Solaster endeca and Crossaster papposus — Gemmill, 1920). The di- 

 gestive tract at this time has only one opening — the blastopore. 



Dipleurula 



Before the formation of the mouth, the ecto- 

 derm of the anterior part of the ventral side of 

 the larva flattens. A small depression forms here, 

 i.e., the stomodeum primordium, and the tip of 

 the gut bends in this direction. Cells of the ec- 

 toderm and endoderm merge and form the mouth 

 opening. The blastopore becomes the anal open- 

 ing and shifts position to the ventral side. The 

 mouth opening is situated in the perioral depres- 

 sion and is edged with a ciliated band (Figure 

 69). This stage is called the dipleurula. In front of the perioral depression 

 rises the preoral plate and behind it, the anal plate with the anal opening. 

 These raised plates are edged with a better developed ciliary cover compared 

 to the remaining surface of the larva. Ciliated cells lying along the edge of 

 the preoral plate later produce the preoral ciliated band, while ciliated cells 

 lying along the edge of the anal plate are the primordium of the postoral 

 ciliated band that extends from the bottom upward, along the dorsolateral 

 margins of the larva. The ciliated bands merge at the anterior end of the larva 

 but later separate here (Figure 70). 



Figure 69: Early 



bipinnaria of Astropecten 



auranciacus (from 



Horstadius, 1939). 



Bipinnaria 



Further development of the trapping and locomotor apparatus — the cili- 

 ated bands — transforms the dipleurula into a bipinnaria. These bands of 

 complex configuration pass along the elongate transparent body of the bipinnaria 

 and over numerous body projections (arms/processes) (Figure 71). 



Feeding : Strathmann (1974) has noted that the body of a planktotrophic 

 larva is primarily a temporary structure for feeding. The digestive system 

 comprises the mouth cavity, esophagus, stomach, a very short, thin intestine, 

 and a thick intestine that terminates in the anal opening. Like the larvae of 

 bivalves, the larvae of sea stars feed on food particles suspended in the 

 surrounding water. Feeding involves the creation of water currents, separation 



