^Auricula´ta^: The larval stage of holothurians. ^Bipinna´ria^ or ^Branchiolaria^: The larval stage of starfishes. ^Dorsal surface^: The back of the animal, generally, but not necessarily, the upper side. ^Exoskeleton^: Outside framework or support, differing from a true skeleton which lies inside the body. ^Interambulacral areas^: The five areas between the ambulacral zones. ^Larva^: The animal in a stage of development from the time it leaves the egg until it reaches the complete form of the species. ^Madreporic plate^: A sieve-like plate of carbonate of lime. ^Oral surface^: The side on which the mouth is placed. ^Ossicles^: Calcareous plates which cover the body and form the exoskeleton. ^Pedicella´riæ^: Small spines which have divided ends, like scissors or forceps. ^Plu´teus^: The larval stage of sea-urchins. ^Po´lian vessels^: One to ten sacs arising from the ring-canal. ^Ring-canal^: The canal around the mouth. ^Spines^: Processes which rise from the surface of the animal. ^Stone-canal^: A calcareous tube leading from the madreporic plate to the ring-canal. ^Suckers^: Sucking-disks on the ends of the tube-feet. ^Ventral surface^: The side opposite the dorsal surface. ^Water-vascular system^: A series of canals which conduct water through the animal. [pg202] ECHINODERMATA The echinoderms, although their shapes are very unlike in the different classes, have the same general internal structure, and also other features which place them together in one group. They are radiates (page 113) of the highest type; they have an exoskeleton; and many of them are beset with spines, from which the name is given. They have locomotor organs, the /ambulacra/, and a /water-vascular system/ peculiar to themselves. Some have the very strange power of casting off, and developing again, parts of the body. The development from the egg to the adult is remarkable, and of unusual interest to naturalists. The larval stage of echinoderms is so unlike the mature animal that for a long time the larvæ were thought to be another class of animals, and therefore were given the names which they still retain. In observing echinoderms it will be seen that the dorsal part is carried uppermost by some, such as the starfishes and sea-urchins. In the former the back is broad and extended; in the latter it is curved and contracted. But in crinoids the back is carried downward and is extended like a stalk; in the sea-cucumbers (holothurians) the dorsal and ventral surfaces are parallel with the long axis of the cylindrical body, instead of being on a plane with the mouth and excretory opening. Owing to the unusual positions of the dorsal and ventral surfaces, the terms /oral/ (mouth side) and /aboral/ (side opposite the mouth) are generally used in describing these species.