832 Marvels of the Universe 
present impossible to determine ; but as the study of shell-life increases, an answer to the riddle 
may not unreasonably be hoped for. We do know, however, the causes which have influenced the 
form of the Watering-pot (4). At a glance, this curious shell can hardly be recognized as a bivalve. 
The rudimentary valves are marked by two scars on the under-side of the tube. The creature that 
inhabited these valves took to burying itself in sand or mud; this caused the valves to curve more 
and more, and as they were never opened (the creature finding ingress and egress from the ends of 
the shell), the edges united and formed a single hollow tube. In this tube it lived, developing all 
the characteristics of the borer shells, 
such as were described on page 696, 
and working its way by means of the 
enlarged foot into the mud and sand. 
But some means was required for 
preventing the sand from falling in 
and filling up the further opening, and 
so the perforated doorway, like the 
rose of a watering-can, was added. 
Possibly the frills with which the 
shell is ornamented are the survivals 
of the corrugations which are the 
familar ornaments of many clasp 
shells, such, for instance, as _ the 
Scallop. 
In certain molluscs perforation in 
the shell will be noticed; some ex- 
amples of such shells are shown in the 
upper illustration on page 831. It is 
supposed that these apertures allow 
the escape of waste material, and in ~ 
some cases the entry of fresh supplies 
of sea-water for the gills of the snail. 
The position of this slit may vary, as 
in the case of the limpets here shown. 
In the Ormer Shell, however, the slit 
is replaced by a series of perforations, 
Skiagraph by] WaGreen™ which run down the whole length of 
SHELL STRUCTURE. the whorls ; as the animal extends its 
Nawtles te buh Noe how he whole cf ake ata maie st gtt shell and makes further perforations, 
central core. This core in shells of graduated whorls, such as cone shells, the earlier holes are closed up. The 
develops into columns; variations from this principle, however, are the A ee 
Staircase Shell and Worm Shell shown on the opposite page. These have ost singular example of the slit is 
1 conte eokyim that of the very rare Pleurotomaria, 
where it forms an open channel through the centre of the later whorls, and can be traced 
throughout the entire coil of the spire. 
The entrance to the snail’s home demands a little attention, for unless it be guarded, it is a source 
of danger to the shell’s owner. Many snails, therefore, adopt the device of the Winkle and make 
horny doors for closing their doorways. This door, or operculum, is attached to the foot, so that 
when it is withdrawn it completely fits into the aperture. All doors are not of the same construc- 
tion ; some are heavy and solid, like polished stone; others are curved and horny, and some are 
shaped just like a screw stopper. But some snails cannot breathe when the door is shut, and 
so these have made a little tubular side-entrance which allows the passage of the necessary 
