1526 THE VOYAGE OF H.M.S. CHALLENGER. 



l^elonging to all families) separated by a clear, rather wide interval, filled up either by a 

 colourless fluid or by a structureless jelly (PI. 103, fig. 1 ; PL 123, figs. 8, 9,&c.). They 

 are in direct connection only at the openings. In the living Ph.^odaria, however, their 

 distance is very small, or they are in immediate contact without any interval (PL 101, 

 fie. 10 ; PL 102, fio". 1). According to the observations of Hertwig, the two membranes 

 are always in close contact, and without interspace, in the living Ph.eodaria ; and the 

 space between them is an artificial product due to the influence of the preserving fluid or of 

 certain chemical agents. In every case it is very easy to separate both membranes 

 completely, except at the openings, where they are in direct connection. "We distinguish 

 both membranes shortly as ectocapsa and endocapsa. 



The ectocapsa, or the outer memlDraue of the central capsule, is rather firm and 

 durable, double-contoured, elastic and difiicult to destroy. Its physical and chemical 

 qualities seem to approach those of chitin. It becomes, however, stained red by carmine, 

 and yellow by nitric acid. Usually it appears structureless and refracts the light strongly. 

 In a few cases, however, it exhibits, when examined by strong lenses, a fine puncta- 

 tion ; and in some Aulacanthida (especially in some big forms of Aulogrcqyhis and 

 AulosiKithis) the entire ectocaj^sa was densely covered with peculiar curved, or S-shaped 

 dark corpuscles (PL 114, fig. 13). They were all of the same length, about O'Ol, and 

 seemed to lie on its inner face. 



The endocapsa, or the inner membrane of the central capsule, is much thinner than 

 the outer, with which it is in immediate connection only at the openings. It 

 encloses the entire contents of the capsule, and becomes very distinct, as soon as the 

 latter are dissolved by chemical agents, or stained by carmine. In the majority of well- 

 preserved preparations it is irregularly plicated, and resembles a thin, but firm, crumpled 

 paper. Isolated pieces of the endocapsa are completely structureless, but exhibit also 

 a considerable resistance, in spite of their minute thickness. 



The openings of the central capsule exhibit in the Ph.eodaria a greater variety 

 than their discoverer, R. Hertwig, supposed. The majority of the legion, certainly, 

 possess the three openings described by him, and are therefore true Tripylea. Some 

 families, however, have only one opening, the astropyle, which is generally present 

 (Challengerida, Medusettida, Castanellida, and Phaiocolla among the Phseodiuida, 

 PL 101, fig. 1). In some other families there is a variable number of accessory 

 openings or parapylse, one, three, or more, e.g., especially in the Circoporida and 

 Tuscarprida. The former may be caUed Astropylea, the latter Sporopylea. Thus 

 only a single opening to the central capsule is constant in aU Ph.eodaria without 

 exception, and that is the astropyle, or the large main-opening with its peculiar 

 structure. 



The astropyle, or the single constant main-opening of the central capsule, is 

 distinguished by a very remarkable structure, and is sufficient of itself to separate the 



