Chap. IV. THAMNOCNIDIA SPECTABILIS. 271 



diameter, and perfectly clear and homogeneous throughout, even to the absence 

 of a mesoblast. What may be, perhaps, called the inner wall proper {Figs. 8, d. 

 and 9, d d^), is a double layer of very large, irregularly polygonal cells, each one 

 of which is nearly filled by a dense mass of dark granular matter (F/ff. 9, d d^). 

 Their distal ends, in the exterior layer, are more or less flattened, and conform 

 to the outlines of the cells of the outer wall (b) ; the proximal ends of those of 

 the inner layer (d) conform to the outlines of the large cells {(/), which fill the 

 centre of the stem. These great cells are very different in appearance and con- 

 tents from those of the double layer which we have just described; some of 

 them, especially those nearest the centre of the stem, are one one hundredth of an 

 inch in diameter; they are perfectly hyaline, and, adding to this their sharp polyg- 

 onal outlines, they have a marked crystalline appearance. Each cell has a single, 

 discoid, homogeneous mesoblast {(/^ g'^), which lies close against the wall.^ The 

 outlines of these cells are gently curved, and form a continuous smooth surface, 

 from one to the other, where they border upon the longitudinal canals. The 

 horny sheath [Figs. 8 and 9, a) possesses very fine concentric lamina?. 



SECTION V 



THAMNOCNIDIA SPECTABILIS AG. 



Proles hi/droidea. Adult. — The description already given of the habitat, the 

 mode of life, the general form, the separate sexes, the head with its proboscis, 

 the tentacles and the bunches of medusoids, the stems and their mode of branching, 

 the horny sheath, the walls of the head and stem of Parypha crocea, apply equally 

 to this Hydroid, with the following exceptions. The horny sheath (PI. XXII. 

 Fig. 16, P) is quite uniform and smooth as far as it covers the stem above its base, 

 and is a very little narrower below than above ; but the entangled mass of the 

 base is perhaps more dense than in Parypha. The medusoids have been observed 

 to be present as early in summer as those of Parypha, but they have been seen 

 much later in the autumn. This difference, however, may be only ap23arent ; 

 perhaps because the two genera were not always collected at the same time ; 

 at least we have no notes indicating that they were. As the various shapes 

 which the proboscis and its tentacles assume in Parypha were not described, but 

 only stated to be identical in that genus and in Thamnocnidia, we will now give 



' The mesoblasts which are represented in this figure all belong to the cells nearest the eye. 



