WEST INDIAN STARFISHES 9 



The mouth is central, dilatable, and surrounded by soft mem- 

 brane. The so-called jaws are merely the adoral ambulacral and 

 adambulacral plates, more or less modified and coalesced. The 

 "teeth" are only slightly modified adambulacral spines. 



The stomach is very saccular and usually evertible. It usually 

 has a lobe and a pair of digestive glands extending into the 

 cavity of each ray, but in the case of some multiple-rayed species 

 (Heliaster) , it has lobes corresponding only to the primary five 

 rays. In this case the five stomach-lobes do not enter the rays, 

 but the five pairs of digestive glands do. In a few slender-rayed 

 genera, also, the stomach is confined to the disk. The intestine 

 is rudimentary or abortive, and not functional in most cases; 

 usually there is a median dorsal nephridial or "anal" pore, main- 

 ly for the discharge of the secretions from a lobulated gland. 



The respiration is partly dermal, but especially by tubular 

 dermal outgrowths, called papula. The madreporic plate is dor- 

 sal, usually single, but sometimes two or more are present in 

 autotomous species. The rays terminate in an enlarged apical 

 or ocular plate, carrying an ocellus. 



The skeleton is made up of large numbers of ossicles and plates 

 of various kinds, mostly articulated so as to be more or less mov- 

 able, giving flexibility, both to the rays and to the disk, though 

 in some species (e. g. certain Goniasteridae) the flexibility is 

 slight, except at the tips of the rays. 



The external skeletal plates are classed as dorsal or abactinal ; 

 marginal; interactinal ; adambulacral; ambulacral; oral; and 

 ocular or apical. The dorsal plates are very diverse in form and 

 arrangement. They may be irregularly reticulate, tesselated, or 

 imbricated. They may form regular radial rows ; sometimes they 

 are abortive, or nearly so. They are concealed by a thick dermis 

 in certain genera. 



The dorsal plates, like the marginals and interaetinals, com- 

 monly bear spines or small spinules, but they may be covered 

 with granules, or with a smooth soft integument, or even appear 

 quite naked, being then covered only with a thin membrane. 



These plates and their armatures of spinules take several 

 special names, according to their forms and structure, and are 

 often characteristic of special genera and families and higher 

 groups. 



