THE CLASSIFICATION OF ANIMALS 57 



but outside the outer wall of the coelom. These plates have 

 resulted from the calcification of the ground-substance of con- 

 nective tissue, i.e. from the deposition in it of salts of lime. 

 As a consequence these plates are porous ; they are indeed 

 scaffoldings rather than solid masses of lime, and in their 

 interstices we find the characteristic amoeboid cells of con- 

 nective tissue. In consequence of possessing such a skeleton, 

 Echinodermata are amongst the commonest of fossils. Their 

 economic importance is extremely slight. The star-fish feed 

 on bivalve mollusca, whose valves they forcibly wrench apart 

 by pulling on them with their suckers ; they then turn their 

 stomachs inside out and digest their victims alive. Star-fish, 

 in consequence, are harmful to oyster-beds. The poorer classes 

 in Italy eat the genital organs of sea-urchins when they are 

 ripe, and to some extent this occurs in our own country also, 

 and to this circumstance the common sea-urchin owes its name 

 of Echinus esculentus, i.e. the edible sea-urchin. In the 

 Malayan Archipelago several kinds of sea-cucumber are fished 

 for and their dried bodies, known as Trepang, are sold to the 

 Chinese who use them for making soup. 



The last phylum which we have to consider and infinitely 

 the most important is that of the Vertebrata, to which we 

 ourselves belong. Metazoa are indeed often divided into 

 Vertebrata and Invertebrata, but this is an unscientific classi- 

 fication and obscures the fact that the Invertebrata which we 

 have been discussing consist of many distinct phyla ; whilst all 

 the animals classed as Vertebrata exhibit the same fundamental 

 type of structure and constitute a single phylum. The chief 

 characteristics of Vertebrata are three, viz. : (1) There is an 

 internal skeleton known as the backbone which runs the 

 whole length of the animal, beneath the central nervous system, 

 but above the digestive tube or gut; (2) the central nervous 

 system is situated near the upper or " dorsal " surface of the 

 animal, and has the form of a tube running along the length of 

 the animal ; (3) the front part of the digestive tube communi- 

 cates with the exterior by means of clefts or pores. In aquatic 

 forms these clefts serve to get rid of the surplus water which 

 the animal swallows with its food ; in these they are termed 

 gill-clefts, but in land forms they exist only in the very young 

 animal and close up when it becomes adult. The word 



