126 KEPORT ON THE PENNATULIDA. 



because it is only then that the amount of lis^ht approximates to what 

 it ia accustomed to receive in the day time at the bottom of the sea. 



The mcliif (Fig 2 u), or axial portion of the feather or polype- 

 bearing; part of the Pen, is widest about the junction of its middle 

 and lower thii-ds. From this pt)iut. at which, in the female specimen, 

 it has a width of 0"2yiu., it tapers j^radually in both directions. Its 

 dorsal surface (Fig. 1) is completely concealed by the polypes, and of 

 the lateral surfaces only small portions are visible between the bases of 

 the leaves. The ventral sm-face is, however, exposed along its whole 

 length (Figs. 2 and 3) ; it is marked by a shallow median longitudinal 

 groove, more pronounced in the female than the male specimen, and 

 is studded all over with the zooids or rudimentary polypes. In colour 

 it contrasts strongly with the stalk, being of a bright red colour, 

 excepting the median groove which is pale yellow. 



The internal structure of the rachis is shown in Fig '6. The central 

 canal, which is of very large size, is divided by the septa shown in 

 this figure into four ; a very large dorsal one, two large lateral ones, 

 and a small ventral one, crescentic in transverse section. In the great 

 size of these canals, which do not appear to have been figured hithex'to, 

 Penitatida phosphorea contrasts remarkably with the allied species 

 Peiinatula rubra, as described and ligured by KoUiker,* in which the 

 dorsal canal is very small, and far removed from the others, which are 

 themselves much smaller than in P. phosphorea. 



The structure of the wall on the mid-dorsal and on the ventral 

 surfaces is, but for the presence of the zooids, much the same as that 

 of the stalk. A single-layered epidermis covers a thick dermis exceed- 

 ingly thickly studded with calcareous spicules, packed together if 

 possible even more closely than in the stalk ; beneath the dermis is 

 a well-developed layer of longitudinal muscles, having the same 

 arrangement as in the stalk ; and underneath this a connective-tissue 

 layer which differs considerably from that of the stalk, for instead of 

 forming a dense compact layer it has the character of a loose spongy 

 meshwork, traversed by large irregular canals and passages, freely 

 opening into one another and into the canal system between the folds 

 of the longitudinal muscles. 



At the sides the structure of the wall between the several polype- 

 leaves is much the same as that just described on the dorsal and 

 ventral surfaces, with the exception that the longitudinal muscle layer 

 is absent, and the spongy connective-tissue layer consequently thicker ; 

 but opposite the bases of attachment of the leaves it is very different. 

 As shown on the left-hand side of Fig. 3, the wall is here reduced to 

 a thin connective-tissue membrane, separating the bottoms of the 

 polype cavities from the main dorsal and lateral canals. 



The partitions separating the main canals from one another are, 

 as in the stalk, formed by prolongations of the connective-tissue layer; 



* KoUiker: Anatomische-systematische Besuhreibung der Alcyonarieii. 

 Erste Abtheilung : Die Peiiuatulideii, iS7-2, Plate VIII., tig. li. 



