30 REPOKT ON tHE PENNATULIDA. 



The central caual is divided into two along the whole length of the 

 stalk by a longitudinal partition ; and in the upper half of the stalk, 

 owing to the presence of two other partitions, into four, whereof one 

 is dorsal, one ventral, and two lateral. 



The walls of the stalk present the following structure : — On 

 the outer surface is an epidermis, which, although of some thickness, 

 consists of only a single layer of closely-packed columnar cells. 

 Beneath this is a thick connective-tissue layer, or dermis, forming 

 from J to ^ the total thickness of the wall. Imbedded in this dermis 

 are an immense number of calcareous spicules crossing one another at 

 every conceivable angle, and set so closely together that in many 

 places the connective tissue matrix is completely concealed by them. 

 These spicules which, unlike the spicules of the rachis and leaves, are 

 colourless, have an average length of O'OlSin., and width of 0-OOlin., 

 the total thickness of the dermis, to which they give considerable 

 strength and toughness, being about 0-016in. 



Beneath the dermis is a well-developed system of longitudinal 

 muscles, arranged so as to form not a simple ring round the stalk, but 

 an extremely sinuous or corrugated one, the loops being very deep and 

 close together, and the total thickness of the layer about J that of the 

 entire wall. Within the layer of longitudinal muscles is a connective 

 tissue layer of varying thickness in different parts, and traversed by 

 ill-defined bands of muscular fibre whose general direction is parallel 

 to the surface of the stalk, though not foi-ming a distinct system of 

 circular muscles. This layer forms also the basis of the septa or 

 partitions dividing the central canal. Finally, the central canals are 

 lined by a single layer of short columnar epithelial cells. 



The walls of the stalk are farther traversed by an in-egular system 

 of canals or vessels of no great size, the largest of which have a longi- 

 tudinal direction and are situated in the loops formed by the layer of 

 longitudinal muscles. 



The lower third of the stalk differs materially in appearance from 

 the upper two-thirds : its walls are softer and paler in colour, and 

 owing to the action of the spirit in which the specimens have been 

 preserved, are very distinctly wrinkled. This difference is due partly 

 to the wall of the lower third being somewhat thinner than that of 

 the upper part, but far more to the fact that in this portion the 

 dermis, which, owing to its calcareous spicules, is the most rigid la5'er 

 o the stalk, is barely half the thickness that it has above. 



We have described the stalk as seen in our spirit-preserved 

 specimens, but before leaving it a point of some interest remains to 

 be noticed. The stalk of Pcimatula phosphorea is described and figured 

 by some writers as of very much greater thickness than we have 

 stated above, and is said to become inflated under certain circum- 

 stances or at certain times of the day. Thus Sir John Dalyell* 



^ Dalvell : " Rare and llcinarkable Animals of Scotland," Vol. ii., 1848, pp. 191- 

 :i)l, and I'latc XLIV. 



