ALCYONTUM. 



519 



Fio. 621. ALCYONIUM : A, portion enlarged, 

 showing the Polypes. 



)ium is a characteristic example. Here the general form and 



aspect 'closely re- 

 semble those of the 

 Sponges; but, in the 

 living state, it will 

 be evident that the 

 projecting orifices arc 

 tenanted by Polypes ; 

 and, even in the dry 

 skeleton, it may ge- 

 nerally be observed 

 that the openings of 

 the large canals are 

 not simple, like those 

 of the vents of Sponges, 

 but present some in- 

 dications of the radiating partitions already mentioned. Between 

 the large canals, into which the Polype-stomachs open, there is 

 a network of minute tubular ramifications, which connect them 

 very intimately. In the interstices of these ramifications, which 

 are occupied by the gelatinous flesh of the animal, spicula, or 

 long needle-shaped crystals, of earthy matter are deposited ; 

 which, as in Sponges, give a general support to the whole mass, 

 though not consolidated in any one point. The general vitality 

 the polypidom, however, would seem to be greater than that 

 Sponge. If a single Polype be irritated, it withdraws 

 )lf within its tube ; and the edges of the cell, which were at 

 first prominent, become nearly flat. If the irritation be more 

 severe, several of the neighbouring Polypes also withdraw them- 

 selves ; and if it be sufficiently prolonged, a collapse and con- 

 traction of the whole polypidom is evident. The analogy is 

 very striking between this phenomenon, and that exhibited by 

 the Sensitive Plant, in which the effects of the irritation are 

 manifested at a greater or less distance, according to its intensity 

 (VEGET. PHYSIOL. 422). If, instead of the Polypes being 

 irritated, a portion of the general surface be touched, a curious 

 series of changes will take place. After some little time the 

 part touched becomes opaque, more dense, and depressed ; and, 



