Page:Scientific results HMS Challenger vol 18 part 1.djvu/824

616 on the surface) the "trizonal shell" is an external or "cortical shell," enclosing the central capsule, the interior of which only contains the simple central chamber and the jointed proximal parts of both lateral wings. In the other genera of Larnacida (by proceeding growth) this trizonal Larnacilla-shell becomes enclosed by the growing central capsule and is now only a "medullary shell," whilst on the outside of the central capsule in the same manner is developed an outer cortical shell (Larnacalpis, Larnacantha); and perhaps the same process may be repeated. But sometimes also this cortical shell becomes doubled by a simple envelop of network (Larnacoma). In the Larnacospongida the cortical shell is composed of a spongy framework (corresponding to the Spongolarcida in the foregoing family).

The lattice-work of the Larnacida is commonly irregular (as in most other ), and its pores have little signification for the different species. On the outer surface often arise radial spines, symmetrically disposed either in dimensive planes or in diagonal planes.

The central capsule is constantly a true lentellipsis or a "triaxial ellipsoid," characterised by three halving, elliptical dimensive planes, perpendicular one to another. It bears a different relation to the skeleton in the two subfamilies of Larnacida. In the first subfamily, the Larnacillida (Larnacilla, Larnacidium), the central capsule encloses only the simple spherical or lentelliptical central chamber ("simple medullary shell"), and is enveloped by the trizonal cortical shell. In the other subfamily, the Larnacalpida (Larnacalpis, Larnacospongus, &c.), this trizonal "Larnacilla-shell" becomes enclosed by the overgrowing central capsule, which now becomes enveloped by an external, latticed or spongy, lentelliptical "cortical shell."