136 MANUAL OF ZOOLOGY. 



ments), " which are disposed radially round a longitudinal 

 axis; always with a determinable, median, antero- posterior 

 and dorso-ventral plane. A distinct body-cavity from which 

 the alimentary canal is entirely shut off, and which, there- 

 fore, never communicates with the outer world through the 

 mouth." 



The Annuloida are divided into two great classes, the 

 Echinodermata 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. In many, if not in all, 

 both neural and haemal systems are developed. 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 

 the body, and are in all cases bilaterally symmetrical. The 



