113 



Figure 73: Development of 

 coeloms in the bipinnaria of 

 Coscinasterias calamaria (from 



Barker, 1978). 



hp — hydropore; Ic — left 



coelom; re — right coelom. 



functional structures performing supportive, dis- 

 tributive, and excretory functions. Coeloms of 

 the bipinnaria are extensive hollow pouches situ- 

 ated on both sides of the digestive tract. In the 

 early bipinnaria they are separate but in the late 

 bipinnaria they fuse above the oral lobe and pass 

 into the dorsomedial projection (arm) (Figure 73). 

 The left coelom is better developed than the right. 

 Septa (commissures) in the posterior region of 

 the coelom partly separate the left and right 

 coeloms (somatocoels) from the common coe- 

 lom. This separation becomes complete later 

 (Figure 74). The remaining part of the coelom is 

 a single structure that later separates into left and 

 right anterior and posterior coeloms, i.e., axo- 

 and hydrocoels. The single coelom is linked with 

 the external medium through a pore canal, which 

 opens as a hydropore on the dorsal side, left of 

 the median line, at the level of the lower part of 

 the esophagus. In some species, in addition to the 

 left pore canal, there may be a right pore canal as well, which also opens 

 through a hydropore exteriorly (Figure 75) (Field, 1892; Gemmill, 1914, 

 1915). During the development of the bipinnaria, coeloms of the right and left 

 side are connected not only in the anterior region, but also in the region of 

 the somatocoels. The left somatocoel temporarily forms a process, termed the 

 ventral horn, which communicates with the right axocoel ventral to the stom- 

 ach, encircling the intestine from both sides. A dorsal horn also forms in the 

 left somatocoel, which establishes contact with the left axo-hydrocoel. Thus 

 the coeloms of the left side remain connected despite a dividing septum 

 (Figure 76). According to Gemmill (1914), the flow of coelomic fluid is 

 directed by cilia from the left axo-hydrocoel to the right axo-hydrocoel, from 

 there through the ventral horn to the left somatocel, and again into the left 

 axo-hydrocoel. In the pore canal, lined with a cuboidal ciliated epithelium, 

 as mentioned above, a weak outward beating of the ciliated band is percep- 

 tible. Possibly, the movement of fluid in the coelomic cavities is facilitated 

 by the pulsation of the closed madreporic vesicle (formed, probably, from the 

 mesenchymal cells), situated alongside the left hydropore (Gemmill, 1914). 

 This vesicle is not found in all species. Nor do all authors mention circulation 

 of the coelomic fluid (see, for example. Barker, 1978). 



Locomotion : Bipinnariae swim by means of ciliated bands, the preoral 

 and, more so, the postoral. The structure of the ciliated bands has been 



