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. " 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, ffivinof 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 — " 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 diff"erent 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." I 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 exchang'ed in the madrecolous tribes for the more 

 perfect defence which a hard coral affords, into which the soft 

 parts are withdrawn at will. " 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. 6.39. t Teale in lib. supra cit. 93. 



J Teale in loc. 95. § Gray in Synop. of British Museum, 70. 



