68 CONNECTICUT GEOL. AND NAT. HIST. SURVEY. [BuU. 



STRUCTURE 



The class Ophiuroidea contains hundreds of species distrib- 

 uted all over the world. Four of these species are known to 

 occur in Long Island Sound, and one other has been found so 

 near the eastern entrance to the Sound that it seems probable 

 that further investigation will reveal its presence in Connecticut 

 waters. 



They all have a flattened, circular or pentagonal body, called 

 the disk, with five very slender, flexible, jointed arms, or rays 

 (Plate XII). These arms taper gradually toward the tip, and 

 their movements in life are such as to warrant the popular 

 appellation, " serpent stars," for, when the animal is disturbed, 

 each one twists in all directions and folds itself about adjacent 

 bbjects in a truly serpentine fashion. 



The mouth is placed in the center of the disk, in a position 

 similar to that in the starfish. Hence the side of the disk con- 

 taining the mouth and the corresponding faces of the arms make 

 up the oral surface, while the opposite side of the disk and arms 

 is the aboral surface. The mouth is provided with five triangular 

 jaws, each situated between a pair of arms, that is, in one of 

 the interradial spaces (Plate XIII). 



The arms are distinctly marked off from the disk, and do 

 not pass gradually into it like those of the starfish. Ambulacral 

 grooves are wanting, and the tube-feet are very delicate. 



The disk contains all the digestive and reproductive organs, 

 the arms containing only a row of calcareous ambulacral ossicles 



Explanation of Plate XIII. Skeleton of Ophiuran. Ophiopholis 

 aculeata. (Four times natural size.) 



Photographs of disk and bases of rays from oral and aboral surfaces 

 to show characteristic arrangement of plates. The upper figure shows the 

 granulations and very blunt spines which cover many of the plates of the 

 disk, leaving only about forty'of the plates exposed. In many specimens 

 the number of exposed plates is smaller; sometimes as few as six are thus 

 left bare. These exposed plates are usually grouped symmetrically in the 

 five radial and five interradial areas, with a single plate in tiie center of 

 the disk. The upper arm plates are seen to be separated by a row of 

 plate-like granules. 



The lower figure shows the five jaws, with four scale-like projections, 

 oral papillae, on each side of each jaw. In the interradial areas are five 

 large plates, oral shields. It will be noticed that the oral shield between 

 the arms nearest the top of the page is larger than any of tiie other four, 

 and forms the madreporic plate. On each side of the ventral arm plates 

 is a single row of flattened spines, the tentacle scales. 



