ANNULOIDA : ECHINODERMATA. 155 



purely provisional arrangement. Many other classifications 

 have been proposed, each with some obvious advantages and 

 some disadvantages. Perhaps the most natural arrangement 

 would be to establish a separate sub-kingdom for the Echino- 

 dermata, and to group the Scolecida with the Anarthropoda under 

 the name of Vermes. In the confessedly imperfect state of our 

 knowledge, however, it will be as well to retain for the present 

 the sub- kingdom Annuloida. 



The Anmdoida are divided into two great classes, the Echino- 

 dermata and the Scolecida. 



CLASS I. ECHINODERMATA. 



The members of this class are known commonly as Sea- 

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

 &c., and the following are their leading characteristics. They 

 are all animals which, in the adult condition, show a more or 

 less distinctly radiate condition of their parts, especially of those 

 around the mouth ; whilst in their embryonic stages they are 

 more or less distinctly bilaterally symmetrical. Whilst radial 

 symmetry in the great majority of cases preponderates in the 

 adult Echinoderm, there are, nevertheless, many instances in 

 which the fully-grown animal shows distinct traces of bilateral 

 symmetry. The external envelope of the body (" perisome ") 

 is either composed of numerous calcareous plates, articulated 

 together, or of a coriaceous integument, in which calcareous 

 granules and spicules are usually developed. In all adult 

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

 lacral 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. The nervous system in 

 all the adult Echinoderms is a ring-like gangliated cord, which 

 surrounds the oesophagus and sends branches parallel to the 

 radiating ambulacral canals. 



The special features of the structure of the Echinodermata 

 will be noticed under each order, but it will be as well to give 

 here an abstract of Professor Huxley's description of the pro- 

 cess of development in the members of the class. In the great 

 majority, if not in all, of the Echinodermata the impregnated 

 ovum is developed into a free-swimming, ciliated, ovoid embryo. 

 Soon the cilia become restricted to one, two, or more bands, 

 which are generally disposed transversely to the long axis of 



