1 THE VOYAGE OF H.M.S. CHALLENGER. 



remarkable aperture. In the case of many and perhaps of all PH^ODARIA the endo- 

 plasm is differentiated into a granular medullary and a thin fibrillar cortical layer, the 

 former of which usually encloses numerous small vacuoles, while the latter contains 

 muscular fibrillse. In the voluminous central capsule of large PH^EODARIA the whole 

 cortical layer of the endoplasm, which lies immediately below the delicate inner 

 capsule-membrane, sometimes appears delicately and regularly striated, and most 

 distinctly so under the apertures, towards the centre of each of which the dark striae are 

 radially directed (see note A, below). These striae are probably contractile muscular fibrillae 

 or " myophanes," by whose contraction the openings are voluntarily widened. In the 

 Tripylea this fibrillar star is much more strongly developed under the astropyle (the main 

 opening) than under the parapylae (or accessory openings); and probably the peculiar 

 radial structure of the operculum of the former is due to the stronger development of 

 these radial fibrils (being their impression). In many PH^EODAKIA, indeed, the fine 

 myophane fibrils are only visible under the apertures, whilst in others they form a con- 

 tinuous fibrillar cortical layer on the whole inner surface of the inner capsule-membrane ; 

 the fine fibrillae run meridionally from one pole of the main axis to the other ; perhaps 

 the whole central capsule may change its form in consequence of their contractions. 

 The medullary portion of the endoplasm, which lies below this thin cortical layer, is 

 usually finely granular in the PH^ODARIA, and permeated by numerous spherical vacuoles, 

 which are noteworthy from their equal size and regular distribution. Each clear 

 vacuole usually contains a dark shining fat-granule, more rarely a group of such 

 granules (see note B). Compare 60, and PI. 101, figs. 1-3 ; PL 104, figs. 1,2; PI. 

 Ill, fig. 2; PI. 128, fig. 2, &c. 



A. The fine fibrillae in the cortical layer of the endoplasm were first described by Hertwig in 

 18*79 (L. N. 33, p. 98, Taf. x. figs. 6-10). He found them, however, only below the three openings 

 in the capsule of the Tripylea, where they form three stellate groups of fibrils. I find them very 

 clearly shown, and with especial distinctness, under the astropyle in most PH^ODARIA, of which I 

 have had the opportunity of examining well-stained and preserved central capsules. In many 

 cases, also, the striation is not confined to the apertures, but spreads over the whole cortical layer. 

 Perhaps this constitutes in all PH^ODAEIA a thin myophane-sheet, whose contractile fibrils run from 

 one pole of the main axis to the other and cause by their contraction changes in the form of the 

 spheroidal central capsule. 



B. The granular medullary portion of the endoplasm of the PH^EODARIA, with its numerous clear 

 spherical vacuoles, was first described in my Monograph (1862), in the case of Aulacantha (p. 263), 

 Aulospheera (p. 359), and Ccelodendrum (p. 361) as a " finely granular, mucous substance (intracap- 

 sular sarcode), packed more or less closely with clear spherical vesicles from O005 to O'OIS mm. in 

 diameter, each of which contains one or two, rarely three, dark shining granules." That these clear 

 spheres are true vacuoles was first clearly proved by Hertwig (L. K 33, p. 98). As a rule all the 

 vacuoles of the same central capsule are of equal size (generally from O'OOS to 0'012 mm. in diameter), 

 and are distributed at equal intervals throughout the finely granular endoplasm. 



