ECHINODERMATA. igi 



An alimentary canal, with or without a distinct anus, but never 

 communicating with the body-cavity. The water-vascular (ambu- 

 lacral] system\often subserving locomotion. Nervous system radi- 

 ate, composed\of an cesophageal ring and radiating branches. 

 Sexes generally distinct, rarely united. 



The members of this class are known commonly as Sea- 

 urchins, Star-fishes, Brittle-stars, Feather-stars, Sea-lilies, Sea- 

 cucumbers, &c. ; and though the fully-grown animal often ex- 

 hibits distinct traces of bilaterality, this is usually more or less 

 completely masked by the general radiate arrangement of the 

 parts of the body. On the other hand, the embryonic Echino 

 derm usually shows distinct bilateral symmetry. The outer layer 

 of the general integument (" perisome ") is ciliated, and the inner 

 layer is more or less hardened by the deposition of carbonate 

 of lime in the form of plates, granules, or spicules. In all 

 adult Echinoderms there is a system of tubes, termed the "am- 

 bulacra! system," which generally subserves locomotion, and 

 usually communicates with the exterior. This water-vascular 

 system surrounds the commencement of the alimentary canal, 

 and in almost all cases gives off secondary vessels in a radiating 

 manner. An alimentary canal is always present, and is com- 

 pletely shut off from the body- cavity. A vascular (pseudo- 

 haemal?) system is generally developed in addition to the true 

 -water -vessels. The nervous system in all the adult Echino- 

 derms is a ring-like, usually gangliated cord, which surrounds 

 the oesophagus and sends branches parallel to the radiating 

 ambulacral canals. 



The process of development is sometimes direct ; but in the 

 typical members of the class a characteristic form of metamor- 

 phosis occurs. The impregnated ovum gives exit to an ovoid 

 embryo or "planula," freely locomotive by means of cilia, 

 which are at first diffused over the body, but which soon be- 

 comes restricted to transverse bands, or to definite outgrowths 

 of the body (" epaulettes ") which are disposed with bilateral 

 symmetry. The larva or " pseudembryo " (fig. 92) next de- 

 velops an alimentary canal, with a distinct mouth and anus, 

 dividing the embryonic body into two bilaterally symmetrical 

 halves. A mass of actively formative protoplasm now appears 

 on one side of the stomach, within which are developed a 

 circular and radial tubes, the whole being the rudiment of the 

 ambulacral system of the future Echinoderm. A symmetrical 

 calcareous skeleton, not converted into that of the adult, may 

 be developed in the larva (as in the Echinoids and Ophiu- 

 roids), or it may be wanting (as in the Asteroids and Holo- 

 thuroids). The mass of protoplasm, above mentioned as 



