THE ANATOMY OP ALOYONIUM DIGITATUM. 373 



water out of the tubes, and causes a considerable contraction 

 of the whole colony. 



If one takes an expanded colony out of the water one notices 

 that, in the first place, the crown and free portion of the 

 polyps are withdrawn ; but when these are completely concealed 

 the colony undergoes still further contraction in size, and water 

 may be seen to be oozing from openings in the centre of the 

 star-shaped tubercles that mark the position of the polyps. 

 This contraction may continue until the colony is reduced in 

 size by about one third of its original bulk, and it is then a 

 great deal harder to the touch than it was when fully expanded. 

 Specimens of Alcyonium in museums and laboratories that 

 have been killed slowly by immersion in weak spirit are always 

 very much contracted in this way, and they offer a marked 

 contrast in consistency to the soft spongy specimens which 

 have been killed suddenly by Lo Biancho^s No. 2 chrom-acetic 

 acid solution. 



A similar contraction of the colony may be brought about by 

 rolling the specimen about in a bucket of sea water. 



This circular muscular sheath may act, then, either as a 

 protractor of the polyps, or, in extreme cases, as a constrictor 

 of the colony. 



The great retractor muscles of the polyps are situated, as in 

 all Alcyonarians, on the ventral faces of the mesenteries. In 

 the regions where the muscles lie, the mesogloea projects in the 

 form of simple or branched lamelliform folds. The muscle- 

 fibres are situated on these folds (fig. 21), and are covered by 

 an epithelium of endoderm-cells. 



In most of my sections it is not easy to distinguish between 

 the muscle- fibres and the processes of mesogloea, as both of them 

 are perfectly homogeneous ; but a double stain obtained by 

 hsematoxylin and eosin differentiates the two tissues mostbeauti- 

 fully, the mesogloea staining blue and the muscle-fibres pink. 



The muscles run longitudinally on the mesenteries, but 

 approach nearer and nearer to the stomodaeum in their passage 

 from below upwards (fig. 14), but their eventual insertion is on 

 the disc close to the mouth. 



