'J18 KEl'OKT ON THK PENNATULII>A. 



cavity. Tiie structure of the calyx is very siuular to that of tlie 

 body-wall, of which indeed it is only the uppermost portion. 

 Each of the eight pointed processes into which it is produced 

 (Fig. 7) is hollow, its cavity (Figs. 10 and 12 h), which is lined 

 by endoderm, communicating somewhat obliquely with the body- 

 cavity. In their upper portions these cavities, like the processes in 

 which they are contained, are situated between the tentacles, as shown 

 in the transverse section drawn in Fig. 12 ; but the lower portions pass 

 obliquely downwards, so as to open opposite the cavities of the tentacles 

 into the chambers between the mesenteries. In Fig. 10 the plane of 

 section passes on the left side of the figure between two of the tentacles, 

 and therefore along the middle of one of the pointed processes of the calyx, 

 the cavity of which is seen in the upper part of the process ; on the right 

 hand side of the figure the section passes down the middle of a tentacle 

 and through the opening of the cavity of a calyx-process into the body- 

 cavity of the polype. 



Each of the calyx-processes is stiffened by one or more calcareous 

 spicules of a very curious shape. These are shown //; situ in Fig. 7 ((') 

 and in Fig. 12, in which latter they are seen cut transversely. Each 

 spicule is a calcareous rod (Fig. 8) about 0.02 inch long, and 0.0009 inch 

 diameter : in transverse section it is, as shown in Fig. [), triradiate with 

 thickened edges. This triple-ribbed form, which is clearly shown in the 

 figures referred to, and which appears to have escaped notice hitherto, 

 is singularly appropriate from a mechanical point of view, forming 

 an admirable combination of lateral strength with lightness of 

 material. Similar spicules, though usually somewhat smaller, are 

 sometimes found in the upper part of the body-wall (Fig. 7). 



The calyx and its processes are devoid of muscles, even the feeble 

 muscles of the body-wall ceasing below the calyx, so that the slight 

 approximation of the points which occurs when the tentacles are 

 retracted must be effected simply by the muscles attached to the bases 

 of the tentacles, the arrangement and mode of action of which we shall 

 notice when describing the mesenteries. 



('. Tlie Te)ttacles a,ve eight hollow * prolongations of the body-wall 

 surrounding the mouth, and fringed on each side by a row of hollow 

 pinnules, usually nine or ten in nnmber. The general characters of the 

 tentacles and their pinnules are shown in Plate I., Fig. 3, and Plate 

 II., Fig. 10, and the microscopic structure in Plate II., Fig. 11, the latter 

 figure representing a transverse section across a tentacle taken about 

 the middle of its loigth and passing through the base of one of the 

 pinnules. 



The tentacles and pinnules being, as before stated, prolongations of 

 the body-wall, consist, like it, of ectoderm, mesoderm, and endoderm ; 

 but the intimate structure and relative proportions of the three layers 

 differ very considerably from those we have found to obtain in the 

 body-wall. 



* Forbes incorroctlv describes the tentacles as solid. Vide .TohiiBtoii ; " British 

 ZoophyteB, " -ind Ed,, l«i7, i'. IGG, 



