8 
Transactions of the 
gravity of the substance of the tube is less .than that of the water 
in which, when liberated, they float. These corrugations are due 
to natural causes, aided by mechanical effects ; they are produced by 
the acts of the animals regressions, which are very frequently in- 
dulged in for periods of some hours’ duration, and which are no 
doubt periods of repose, when beyond the mechanical effect of such 
a contracted position there is possibly a partial arrest in the secre- 
tion of the substanco of the theca. 
The theca of the Family Melicertad.e, on the other hand, is 
never organically attached after it is once deposited ; the gelatinous 
basis of which this foundation is formed is secreted by a secerning 
gland, situated in the anterior region, and in a neural aspect, in 
close proximity with the ganglion, and it is discharged from an 
orifice in the centre of the jiguruin (Plate XXIV., Figs. 2, 3, r). 
This cup-like organ is present in every individual of this Family, 
and is furnished with cilia in every species that attaches any 
foreign substance to the theca. It is, however, unciliated in Meli- 
certa annulatus, where the theca is free from any extraneous acces- 
sion except such as become attached parasitically. In M. pilula it 
is also unciliated, for here the animal fortifies the gelatinous basis 
with her own voided excremental pilules. And in Lacinularia 
volvox, that free roving cluster, it is also unciliated where the theca 
is always devoid of any foreign accession. 
There are two species in which the theca is constructed upon a 
symmetrical plan, Melicerta ringens and M. annulatus, and it 
attains a high degree of perfection in M. ringens ; and I take this 
opportunity of recording that it has been my privilege to discover 
the particular organs by whose functions the mathematical precision 
with which the pellets are deposited upon the theca is governed. 
They have their homologue in M. annulatus, the only other species 
that constructs its theca on a symmetrical plan. I refer to the 
two “ hooked spines ” situated on the haemal side of the neck, which 
are the most conspicuous organs on the emergence of the animal, 
and these have been erroneously called “ bps.” I must, however, 
content myself with stating that, notwithstanding their being 
situated on the haemal side, they are employed in governing the 
mathematical precision with which the pellets are deposited on the 
opposite side; and in this brief review I confine myself simply to 
recording the fact. A more detailed description will be given in 
the ‘ Memoir ’ on which I am at present employed. 
In the particular instances of Melicerta ceratophyXli, M. crystal- 
linus, and M. intermedins, the theca is formed of a consistent shape, 
and with well-defined walls, leaving a clear space for the motions of 
the inhabitants, but without any system of parts symmetrically 
arranged, and with these, the upper portions are more densely 
blended with foreign accessions than the lower; while with M. 
