ZOOPHYTA HELIANTHOIDA. 
199 
of a prodigious number of vessels, wherein a liquor is seen to 
circulate. The largest of these unite together, much in the 
same manner as the optic nerves do in man.” # 
The external envelope of the naked Hydroida is a thick firm 
fleshy or coriaceous skin consisting of a corium and epidermis, 
— the former layer constituting the chief organ of support, and 
giving to the animal its peculiar form. 44 A circular horizontal 
portion forms the base or foot ; a cylindrical vertical portion 
constitutes the sides, and is inflected at the superior border, so 
as to form a thick rounded lip. The corium is afterwards pro- 
longed over the tentacula, giving investment and form to these 
organs, and is then extended horizontally to form the roof, near 
the centre of which it again becomes folded upon itself, forming 
an internal lip or mouth, at which part it is continuous with the 
digestive sac.” f— 44 The epidermis forms a thin layer of unor- 
ganized matter spread over the whole extent of the corium, and 
may be traced into the stomach. The external surface of the 
epidermis is dense and membranous ; internally, when examin- 
ed by the lens, it appears as a pulpy substance. Intimately in- 
termixed with it, in irregular patches, and not constituting a dis- 
tinct or separate layer, is a pigment varying in colour in different 
parts of the same animal, and in different individuals. This 
colouring matter is extensively distributed over the base, sides, 
tentacula, and roof, but I have never observed any trace of it in 
the stomach.” J The surface is either smooth or studded over 
with glandular warts, which, having an adhesive quality, enable 
the creatures the more completely to conceal themselves by in- 
duing the body with an extraneous coat formed of the sand, 
gravel, and broken shells which lie around their peculiar locali- 
ties. This is exchanged in the madrecolous tribes for the more 
perfect defence which a hard coral affords, into which the soft 
parts are withdrawn at will. 44 This coral is calcareous, and the 
cells which are inhabited by the animals are furnished with more 
or less distinct longitudinal lamellae, placed in a radiating posi- 
tion round the central axis, so as to give the cavity a star-like 
appearance.” § Its structure is in fact a model cast in lime of 
what may be called the skeleton of the Actiniae, — the parts on 
* Phil. Trans, abridg. xiii. 639. f Teale in lib. supra cit. 93. 
t Teale in loc. 95. § Gray in Synop. of British Museum, 70. 
