In the genus _Palmipes_ we have the pentagonal form, it is true, but the space between each ray is filled up, so as to resemble the webbed foot of a bird, hence the popular title of this solitary species, 'The Bird's-foot Sea-star.' 'It is the flattest of all its class, and when alive it is flexible like a piece of leather.' Passing by the 'Cushion-stars' (which have five _angles_--it seems a misnomer to call them rays), which connect the true Star-fishes with the Sea-Urchins, we come lastly to the 'Lingthorn,' _Luidia fragillisima_, with its seven rays. This is the animal of which Professor Forbes discourses so pleasantly about its winking derisively at his despairing endeavours to preserve even a small portion of what at that time was his maiden specimen. The Luidia is even more brittle--more regardless of its wholeness, than the _Ophiuræ_, which renders the capture of a perfect specimen a most difficult task. CHAPTER XIX. Sea-Urchins (Sea-Hedgehogs). 'Truly the skill of the Great Architect of Nature is not less displayed in the construction of the Sea-Urchin than in the building up of a world.'--P. FORBES. [Illustration: 1 THE APLYSIA or SEA-HARE 2 PURPLE-TIPPED SEA-URCHIN 3 Spine of PURPLE-TIPPED SEA-URCHIN 4, 5 Suckers of PURPLE-TIPPED SEA-URCHIN 6 COMMON SUN-STAR] XIX. Sea-Urchins are frequently taken in dredging. Several common species, usually of a small size, are often found among the rocks situated between tide marks. Into the aquarium no specimens larger than from one to two inches in diameter should be introduced, and even these require to be closely watched, for if afflicted with a fatal illness, I know of no animal whose remains sooner taint the water. Almost before life is extinct, the Urchin throws out a light-coloured nauseous fluid, that speedily poisons the surrounding water, and, of course, causes the destruction of any inhabitants of the tank who may neither have the sense nor opportunity to inhale copious draughts of fresh air. As a rule, if the suckers are motionless, or if on touching the animal it is found not to be adherent to any object, transfer it at once to your 'infirmary' for further observation. I have always found small specimens to be much more lively and walkative, (if I may be allowed the expression) than their more corpulent brethren.