420 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



When removed from the calyx the visceral mass is a roundish ball from a 

 quarter to half an inch in diameter and somewhat flattened on the ventral surface. 



The dorsal surface, which normally rests on the calyx, is covered by a compact 

 layer of connective tissue which has a silky appearance on the outside. In the 

 center of the dorsal surface is a minute perforation which serves for the passage 

 of the central plexus from the chambered organ into the visceral mass. The 

 ventral surface is covered with epidermis overlying connective tissue, continuous 

 with a similar covering on the ventral surface of each arm. 



In some specimens of Antedon bifida the visceral mass is very easily removed 

 from the calyx by pushing it aside with a blunt needle, but it is a noteworthy 

 fact that this evisceration can not be effected with equal ease in all cases, for 

 while in some specimens the visceral mass comes away almost with a touch, in 

 others it adheres firmly to the calyx and can not be brought away at all except 

 by tearing it all to pieces. 



While at Millport he observed that specimens were dredged which had 

 already lost their visceral masses. At first he was inclined to think that the 

 animals cast them out of their own accord on being irritated, but this must 

 be regarded as very doubtful, for owing to the slight manner in which they 

 are generally attached to the calyx, the visceral masses might have been torn out 

 by rough treatment in the dredge. 



It was observed that the naturally eviscerated specimens had the genital 

 products more or less undeveloped, while those with the pinnules distended had 

 the visceral mass more firmly attached to the calyx. Dendy supposes that the 

 necessity for uninterrupted nutrition at the breeding period may account for the 

 greater tenacity of the visceral mass during this time. 



Removal of the visceral mass appears to affect the animals but little. At 

 first they fold their arms over the calyx, but this condition does not generally last 

 long, and in an hour or two, when they have once recovered from the shock, their 

 vigor seems to be unimpaired. 



It is evident that evisceration involves tearing of the epidermis and dermis 

 in a more or less regular circle all around where the arms join the disk, and 

 that the ambulacral grooves and nerves, together with the ambulacral vessels, must 

 also be torn across. The central plexus must also be broken across. 



Isolated specimens of the visceral mass of Antedon bifida were observed by 

 Sir Wyville Thomson to perform slow creeping movements, but Dendy noticed 

 nothing of the sort; the visceral masses appeared to remain quite still and to 

 slowly decay. 



The central capsule, together with the nerves radiating from it to the arms, 

 is uninjured by the operation. The line of tear along which the oral perisome 

 has been separated from the integument of the arms is very distinctly marked; 

 each ambulacral groove is torn sharply and suddenly across, generally at about 

 the level of the base of the oral pinnules. 



Between any two adjacent arms or pairs of arms is a tightly stretched trans- 

 lucent membrane bounded by a free incurved edge which marks the line of tear 

 interradially and radially. These membranes form an important part of the floor 



