694 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



smoothing them down with the fingers and pressing out the curves, results in per- 

 fectly expanded specimens. 



Only in relatively few cases, and these were individuals which had been in the 

 laboratory for some hours, was the muscular contraction great enough to break 

 off the arm or even cause loss of cirri. These cases are probably to be explained 

 by the susceptibility of these animals to nonaerated sea water that is, presumably, 

 to carbon dioxide (CO 2 ). It was not possible to determine positively whether 

 carbon dioxide was the deleterious substance or not, but there is good 

 reason for that belief, because the comatulids were found only on those parts of the 

 reef where the aeration of the sea water was very well provided for by the surf. 

 Whether carbon dioxide was the factor directly responsible or not, it is 

 beyond question that the comatulids brought to the laboratory were very susceptible 

 to the impurity of the sea water after a few hours in the aquaria. 



Speaking of Tropiometra picta as he found it at Buccoo Bay, Tobago, Doctor 

 Clark says that indifference to the amount of carbon dioxide in the water 

 was noticeable, particularly in view of the sensitiveness of the Torres Strait coma- 

 tulids to that substance. 



Two specimens lived over night in only 2 liters of sea water apparently without 

 discomfort, and four or five specimens were often kept over night in aquaria with 

 not more than 6 or 7 liters. 



This species responded to a saturated solution of magnesium sulphate in sea 

 water by a complete relaxation of the muscular system. The response was much 

 less complete if the individual had been for some time in a limited amount of sea 

 water ; probably the presence of carbon dioxide made the difference. 



Response to alcohol and formalin was, as in the case of the Torres Straits 

 comatulids, by flexion of the arms first dorsally and then ventrally. Only excep- 

 tionally, and then by individuals which had been long in tank or aquaria, were 

 the arms cast off. 



RESPONSE TO MECHANICAL STIMULI. 



Dr. W. B. Carpenter noted that in Antedon bifida if the pinnules other than 

 the oral pinnules of any arm be irritated by the contact of a rod such irritation 

 merely produces a languid wavy motion of the arm thus acted on, which may 

 extend itself to others if the irritation be repeated or prolonged. But if the rod 

 be made to irritate the long proximal pinnules all the arms, if the animal be in 

 full vigor, immediately close together with an energy and consentaneousness that 

 are seen in no other movement. 



Bohn, experimenting with Antedon mediterranea at Banyuls-sur-Mer, found 

 that when either the disk or the cirri are touched, simultaneous movement of the 

 five arms and their branches occurs. 



On every stimulation of the disk the arms are raised, forming a sort of dome 

 above it. 



On every stimulation of the cirri the arms are lowered. 



Thus, as a general rule, there occurs flexion toward the pole touched. The 

 intensity of the reaction is variable. The firmer the attachment of the animal the 



