192 



THE ATLANTIC. 



[chap. IV. 



behind the anterior border of the ambulatory tract, and a little 

 in advance of the posterior extremity of the body respectively. 

 A slightly elevated pyramid of five very accurately fitting cal- 

 careous valves closes over the oral aperture and the ring of oral 



tentacles ; and a less regular valvular 

 arrangement covers the vent. 



In the middle of the back in the 

 female there is a well-deiined saddle- 

 like elevation formed of large tessel- 

 lated plates somewhat irregular in 

 form, with the surfaces smoothly gran- 

 ulated (Fig. 39). On removing one or 

 two of the central plates, we iind that 

 they are not, like the other plates of 

 the perisom, imbedded partially or al- 

 most completely in the skin, but that 

 they are raised up on a central col- 

 umn like a mushroom or a card-table, 

 expanding above to the form of the 

 exposed portion of the plate, contracting to a stem or neck, and 

 then expanding again into an irregular foot, which is imbedded 

 in the soft tissue of the perisom. The consequence of this ar- 

 rangement is that when the plates are fitted together edge to 

 edge, cloister-like spaces are left between their supporting col- 

 umns. In these spaces the eggs are hatched, and the eggs or 

 the young in their early stages are exposed b}'' removing the 

 plates (Fig. 40). At first, when there are only morales or very 

 young embryos in the crypts, the marsupium is barely raised 

 above the general surface of the perisom, and the plates of the 

 marsupium tit accurately to one another; but as the embryos 

 increase in size, the marsupium projects more and more, and 

 at length the joints between the plates begin to open (Fig. 

 39), and finally they open sufliciently to allow the escape 

 of the young. The young in one marsupium seem to be all 



Fig. 39. — PsoUis ephippifer, Wy- 

 viLLE Thomson. Coriuthiau Har- 

 bor, Heard Island. Three times 

 the natural size. 



