PENNATULA PHOSPHOREA. 89 



calices, they are far more numerous than iu other parts of the polypes, 

 being set so close together as to be almost in contact with one another; 

 they are also, as we have seen, exceedingly abundant in the dermis 

 of both stalk and rachis. 



The spicules, which are always mesodermal structures, vary much 

 in size in different places. They are straight rods, about twenty times 

 as long as they are wide : in the polypes the smallest spicules have a 

 length of about O-OOoin., while the largest ones measure O-O-tliiu. long 

 by 0"002in. wide, the average length being about O'OlSiu. The 

 transverse section varies in shape according to the size of the spicules ; 

 the smaller spicules are, as shown in Fig. 0, vei\y distinctly triradiate, 

 but of a heavier and less elegant pattern than in Fioiiculinci (r/, Plate 

 I., Fig. fl). In the larger spicules the grooves between the ribs are 

 filled up more or less completely, as shown in Fig. 7, while the largest 

 spicules of all have entirely lost the triradiate character and are 

 circular in section. This relation between the size of the spicules and 

 their shape in tranverse section appears to be a very constant one, so 

 that for each length of spicule there is a characteristic shape in 

 section, which is rarely departed from to any considerable extent. 



The spicules are not unfrequently rather wider in the middle than 

 towards the ends, which latter are slightly rounded off. As already 

 stated, the spicules are bright red, the red colour of the leaves and 

 rachis being due entirely to them. 



<■. The Teiitacle-'i, as in Funiculina, are eight hollow processes of the 

 body-wall placed round the mouth, and bearing on each side a row of 

 from ten to fifteen hollow pinnules (Fig. 6, /). The tentacles are shown 

 in transverse section near to tlieir bases in the uppei'most section of 

 Fig. 5, which shows their structure at this part. Each consists of an 

 outer layer or ectoderm, with abundant thread-cells or nematocysts ; 

 an endoderm lining the central canal, and continuous with the 

 endodei-m of the body-cavity ; and a mesoderm, which at the sides 

 and inner surface of the tentacle is thin, as in Funiculina, and 

 consists of a layer of longitudinal muscles with an inner much 

 weaker layer of circular muscle-fibres. At the outer side of the 

 tentacle the mesoderm (Fig. 5) is very much thicker, and resembles in 

 structure the mesoderm of the body-wall, consisting, in addition to an 

 outer layer of longitudinal muscles, of a gelatinous connective-tissue 

 matrix in which are embedded a number of calcareous spicules 

 (Fig. 5, /). 



The pinnules are at the lower end of the tentacle rather long, 

 thin, and some distance apart ; towards the upper end they become 

 thicker and more closely set together. Their cavities open into the 

 central cavity of the tentacle, and their structure is the same as that 

 of the tentacle itself : they may even contain small calcareous 

 spicules. 



(I. The Stomacli, as seen in Figs. 4 and 5, is very similar to that of 

 I'uniculind. It is short, and is entirely contained in the free portion of 



