Both the Hexasterophoran genera were dredged off the north of Scotland, and both conform to the Lyssacine type without uncinates. _Euplectella suberea_ is a straight, erect tube, anchored by a tuft {206}of basalia. The upper end of the tube is closed by a sieve plate, the perforations in which are oscula, while the beams contain flagellated chambers, so that the sieve is simply a modified portion of the wall. It is a peculiarity of this as of one or two other allied genera that the lateral walls are perforated by oscula. They are termed parietal gaps, and are regularly arranged along spiral lines encircling the body. [Illustration: FIG. 99.—Sieve plate of _Euplectella imperialis_. (After Ijima.)] Ijima, who has dredged Euplectellids from the waters near Tokyo, finds that in young specimens oscula are confined to the sieve plate; parietal gaps are secondary formations. The groundwork of the skeleton is a lattice similar to that shown in Fig. 100. The chamber-layer is much folded. Various foreign species of _Euplectella_ afford interesting examples of association with a Decapod Crustacean, _Spongicola venusta_, of which a pair lives in the paragaster of each specimen. The Crustacean is light pink, the female distinguished by a green ovary, which can be seen through the transparent tissues. It is not altogether clear what the prisoner gains, nor what fee, if any, the host exacts. Ijima relates that the skeleton of _Euplectella_ is in great demand in Japan for marriage ceremonies. He also informs us that the Japanese name means "Together unto old age and unto the same grave," while by a slight alteration it becomes "Lobsters in the same cell," and remarks that the Japanese find this an amusing pun. [Illustration: FIG. 100.—Skeletal lattice of _Euplectella imperialis_. (After Ijima.)] The same _Spongicola_ lives in pairs in _Hyalonema sieboldi_. Another case of apparently constant association is that of the Hydroid stocks which inhabit _Walteria_. F. E. Schulze describes _Stephanoscyphus mirabilis_ (see p. 318) in a specimen of _Walteria flemmingi_; the presence of the polyp causes the sponge to grow out into little dome-shaped elevations, each of which shelters one polyp; while in _W. leuckarti_ Ijima finds a similar association in every specimen examined. {207}FOSSIL HEXACTINELLIDA. This group has the distinction of including among its Lyssacine members the oldest known sponge, _Protospongia fenestrata_, of Cambrian age (Salter). As preserved it consists of a single layer of quadriradiate, or possibly quinqueradiate spicules, which, arranged as a square meshed lattice, supported the superficial layer of the sponge (Fig. 101). Whether or not the fossil represents the whole of the sponge-skeleton does not appear.[236] [Illustration: FIG. 101.—Part of the specimen of _Protospongia fenestrata_ in the Sedgwick Museum, Cambridge. Nat. size. (After Sollas.)]