908 REPORT—1850. 
of neighbouring sacs, while in others this communication is interrupted by 
the existence of septa, to be presently described. 
The structure of the endocyst in all the six genera is cellular, and in all 
cases admitting of favourable observation ; transverse muscular fibres, to be 
afterwards described, may be detected in it. Shortly after its inversion it 
becomes altered in texture, losing its contractility and assuming a thinner 
and more membranous appearance ; in this condition it continues till it ter- 
minates by being attached to the base of the tentacular crown; this thin non- 
contractile portion of the endecyst constitutes the tentacular sheath which 
encloses and protects the tentacula during the retracted state of the poly- 
pide. A portion, perhaps the whole, of the inner surface of the endocyst is 
clothed with vibratile cilia. 
The ectocyst, or external investment, is in most of the species composed of 
a tough pergamentaceous brown membrane, strengthened by the deposition 
of irregularly-formed siliceous particles, which, except towards the apertures, 
where these particles are deficient, give to the ectocyst an opacity which ren- 
ders an observation of the contained parts a matter of considerable difficulty. 
In some species of Plwmatella and in Alcyonella flabellum, and A. Benedeni, 
the earthy particles are entirely absent from a longitudinal line which com- 
mences wide near the aperture of the cell, and gradually narrows as it passes 
downwards, when it soon assumes the appearance of a prominent keel, and 
then loses its transparency by the deposition of earthy matter, as in the rest of 
the tunic. The perfectly transparent wide origin of this line gives to the 
orifice of the cell the appearance of a deep notch at one side. In Frederi- 
cella a slightly prominent keel is also apparent, but the netch-like transparent 
space does not exist. 
In Cristatella the ectocyst would at first sight seem to be entirely absent, 
and the eccencecium to be composed exclusively of the endocyst. A careful 
examination, however, proves that both are present, and that the ectocyst 
consists of a highly-organized flexible and transparent tunic, of very evident 
cellular structure, and quite free from every kind of earthy deposit. The 
whole of this tunic is contractile, and presents below a flattened dise desti- 
tute of apertures. Upon the disc, which closely resembles the foot of a 
gasteropodous molluse, this singular colony creeps about upon the stems and 
leaves of aquatic plants, exposing its beautiful plumes to the light and warmih 
of the sun. 
Lophopus also at first sight conveys the impression of being destitute of an 
ectocyst, and having the place of this tunic supplied by a peculiar unorganized 
gelatinous secretion, in which the colony is enveloped. This apparently 
gelatinous investment is however in reality a distinctly organized tunie, which 
seems formed of a cellular or areolar tissue, enclosing in its meshes a transpa- 
rent and colourless fluid. That such is its structure becomes apparent when 
the animal has undergone partial desiccation after removal from the water, for 
then the ectocyst loses a portion of the fluid which had been imprisoned in 
its tissue, and its membranous nature becomes revealed. Neither Trembley 
nor Baker takes any notice of this gelatinous envelope. M.Dumortier men- 
tions it, and represents it in his figure*, while M. Van Beneden believes it to 
be an accidental investment acquired by the animal during confinementt. 
The ectocyst in Paludicella is formed of a smooth pergamentaceous semi- 
transparent membrane, free from earthy deposit, and of a deep brown colour. 
Towards the orifice of the cell it becomes thin and delicate, and is here 
strengthened by four longitudinal horny ribs. The part of the ectocyst to 
which the ribs are attached is carried inwards during extreme retraction οὗ 
* Dumortier, doe. cit. + Van Beneden, (oc, cit. 
