120 



brachiole, the medioventral process in sea stars of the order Spinulosida (for 

 example, Acanthaster planci — Henderson and Lucas, 1971; Patiria pectinifera 

 — Mortensen, 1921; Kasyanov, 1977) is subjected to less change than in sea 

 stars of the order Forcipulatida (for example, Asterias rubens — Gemmill, 

 1914; Asterias amurensis — Kume and Dan, 1968). The axocoel processes 

 enter the brachiolar arm. The preoral ciliated band extends laterally on each 

 brachiolar arm. The brachiolar arms are crowned by a ring of attachment 

 papillae which serve in probing the substrate and temporary attachment to it 

 (Figure 81). Neurosecretory and sensory cells are present in the papillary 

 epithelium; the secretory cells predominate and produce a mucopolysaccha- 

 ride-type secretion. Each such cell is armed with cilia and encircled by a ring 

 of microvilli. Another type of secretory cell is rarely found, namely, one 

 devoid of cilia but covered with numerous microvilli. The characteristic 

 feature of the so-called vacuolated cells (possibly, these are actually secretory 

 cells partially devoid of secretion) is intracellular fibrils, which extend from 

 the basal part of the cell into the microvilli. The function of these structures 

 is supportive. The lateral papillae, situated on each side of the attachment 

 disk, are identical in structure. 



The attachment disk is a round, slightly concave structure comprising 

 secretory cells covered with numerous branched microvilli. The proteinic secre- 

 tion of these cells acts as a cement for attaching the disk to the substrate. As 

 in the papillary cells, supporting fibrils are located in the disk cells (Figure 82). 



While probing the substrate, the brachiolaria aligns itself by its ventral 

 side, flexes its arms covered with a ciliated band, and attaches itself by means 

 of one or two brachiolar arms contacting the substrate with their papillary 



Figure 80: Attachment system of the 

 brachiolaria of Coscinasterias calamaria. 

 ad — attachment disk; b — brachiolar arm. 



Figure 81: Brachiolar arm of the 

 brachiolaria of Patiria pectinifera. 



