WEST INDIAN STARFISHES 81 



surface covered by a supradorsal membrane supported by the 

 tips of long, slender, divergent, often webbed, paxillary spin- 

 ules and pierced by small concentric pores or "spiracles" often 

 closed and invisible in preserved specimens and usually with a 

 central osculum. Beneath this membrane is a "nidamental cav- 

 ity" or gonocodium, traversed by the columnar pseudopaxillae, 

 and containing the papulge. In this cavity the eggs are retained, 

 and also the young, till they assume the adult form, often 8 to 

 jQmm jjj^ diameter. The dorsal skeleton ossicles are lobed or cruci- 

 form, loosely reticulated. 



The adambulacral spines usually form transverse webbed 

 combs or fans (not much webbed in Hymenasterinse). 



Series of slender, divergent spines more or less appressed 

 and attached to the under surface, or imbedded in it, and us- 

 ually webbed to the adambulacral fans in PterasteriuiE, are al- 

 ways present. These peculiar spines, called ' ' actino-lateral 

 spines" by Sladen, should rather be called retroamhulacrals or 

 outer adambulacrals, for they are attached to the outer end of 

 the adambulacral plates. They are sometimes short, and do not 

 reach the margin, except distally, on the rays, but in Pteraster 

 and some other genera their tips usually form the marginal edge 

 of the disk. 



Between the bases of these there is usually a small slit or pore 

 (actinal spiracle or ''segmental aperture") furnished with a 

 partly calcareous valve. Interactinal plates are lacking. Pedi- 

 cellariae have not been found. Jaws have a series of adoral 

 spines, usually webbed. Epioral spines of large size, in one to 

 three pairs, often without webs, stand perpendicular to the jaw ; 

 those of one pair are sometimes specialized and partially hyaline. 

 Ambulacral feet are large, in two to four rows, with large apical 

 suckers. 



The supradorsal membrane may be thin and translucent, with 

 thin muscular fibres, or thick and muscular. In some cases it 

 contains calcareous spicules, muscular and cartilaginous fibers 

 in the form of a network, and often abundant mucous glands. 

 The copious mucus is phosphorescent in some cases (Diplop- 

 tereaster.) The intestine and anal pore are well developed. Sex- 

 es are alike externally. 



