MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 85 



The usual color of the species is an olive green, with darker bands on the arms and sometimes 

 with a clouded disk. 



Through the blending of their colors with the seaweed the ophinrans arc greatly protected 

 from their enemies, and it is difficult, even when looking for them, to see them among the seaweed 

 so long as they do not move. 



It is quite common to find a small Amphipod crustacean clinging to the arms of dredged 

 specimens, and from the structure of the crustacean it is probable that the two species live 

 together commensally. What benefit either animal can derive from the association it is difficult 

 to see. 



One pair of the thoracic legs of the crustacean is so modified as to form a structure beauti- 

 fully adapted tor clinging to the round ophiuran arms. The last segment but one of each of this 

 pair of legs is Y-shaped. At the end of oue arm of the Y is attached a movable segment, the end 

 segment of the leg, which when shut down upon the end of the other arm of the Y incloses a 

 triangular space in which the ophiuran arm is held. 



The body of the crustacean is colored and banded in such a manner as to simulate closely the 

 color and banding of the ophiuran arms. 



When placed in aquaria with their host, the crustaceans cling to the ophiuran arms until the 

 water becomes depleted of oxygen, when they leave the arms and swim about the edge of the dish 

 apparently much alarmed. 



In examining the stomachs of the ophiurans one finds bits of other animals, such as crustacean 

 appendages and the skeletons of young horseshoe crabs. From this it is probable that the crea- 

 tures are scavengers, since an active crustacean would hardly be captured by so slow and poorly 

 armed an animal as an ophiuran. None were ever observed to eat anything when kept in the 

 laboratory, and it is quite out of the question to observe them in their natural habitat, since they 

 are nocturnal animals remaining hidden during the day. 



The ophiurans were first examined for sexual elements early in June, and at that time the 

 eggs were very large but adhered closely together in the gonads. The sperm appeared to be fully 

 formed but were nonmotile. 



From this time on until the middle of August the species was regularly watched and exam- 

 ined, aud on July 16 the first ripe eggs and sperm were obtained. A great number of specimens 

 had that day been dredged and placed in aquaria dishes of fresh, filtered sea water. One week 

 later a great number of adults were again brought in and placed under the same conditions as 

 those which had spawned in the laboratory the week before, but this time very few eggs were 

 obtained, and all subsequent attempts to get the ophiurans to spawn were unsuccessful. 



From this it would seem that the breeding season is extremely short. 1 



The time of day at which spawning occurred corresponds well with the time at which 1 have 

 noted it to take place in OpMopMlus aculeata and Ophiocoma eehinata, that is, between S and 10 

 o'clock p. m. 



PHYSIOLOGICAL NOTES. 



The locomotor movements of an ophiurid, upon a casual observation, seem to consist of an 

 uncoordinated writhing and twisting not calculated to bring the creature to food or a place of 

 safety except by chance; but a more careful study shows them to be the result of an orderly and 

 nicely coordinated mechanism. 



The rapid strides which characterize the movements of a brittle star are in strong contrast 



1 During the summer of 1899, after this paper hail gone to press, my experience with the species was very dif- 

 ferent from the above. Specimens brought into the laboratory early in June threw eggs and sperm, but the eggs, 

 after passing through the early segmentation stages, ceased to develop. The eggs were probably immature, and 

 were spawned only because of the bad condition of the water in the aquaria, hut spawning always occurred early 

 in the evening at the time when it would have occurred under normal conditions. Why unripe e^gs should develop 

 at all, or why eggs mature enough to begin their development should not be mature enough to complete it, is an 

 interesting question. 



This phenomenon was repeated every few days until July 26, when about one-fourth the number of eggs 

 spawned developed into normal larva-. This is ten days later than the date when eggs became' mature at Woods 

 lloll. From the fact that the water is much warmer at Ueaufort than at Woods Holl oue would expect to find the 

 spawning season earlier at the latter place. 

 10396 2 



