CLASSIFICATION OF THE ANIMAL KINGDOM. 99 



the egg, ave not fully organized, but they undergo several metamorphoses 

 before assuming their perfect shape. No circulatory or respiratory system 

 has as yet been discovered in them; and it is in this respect that, though 

 forming a gradual transition to the articulated animals, their organization 

 must be considered decidedly inferior. 



We now come to the last tribe of the Nematoneurose division, the 

 Echinodermata — animals of an exceedingly interesting structure, which 

 approximate intimately, says Professor Jones, with the Polyps on the one 

 hand, and the annilose animals on the other. The first tribe mentioned 

 is that of the Crinoidea — animals which are attached to rocks by a pedicle 

 or foot-stalk, and secrete a stony skeleton in all their parts. They are 

 abundant in the fossil-world, but very rarely met with alive. Nothing is 

 known more of their structure than that they possess a mouth and canal 

 aperture, and we have only one minute species in our own seas. 



The Asteridce come next, and in their lowest structure resemble some- 

 what the last tribe of animals, only they are not fixed on a pedicle, 

 but can move about at pleasure. Passing by the Ophiura to the true 

 Star-fishes, we come to animals possessed of a central disc of a wonderful 

 and beautiful construction, and a variable number of rays, into which, in 

 the largest species part of the viscera extends. The mouth of these 

 animals occupies the centre of the ventral surface, and, on the under side 

 of each ray, we find immense numbers of tentacula or suckers, called 

 ambulacra, arranged in parallel rows, which can be protruded at pleasure, 

 and are used both for the purposes of locomotion and as instruments for 

 the prehension of food. 



The next type we meet with of this class, that of the Echinidce, has no 

 longer separate rays, though their form is still preserved in the flat or 

 globose animals which this tribe includes. The Holothuridce and Fistula- 

 ridm come next — animals which unite in a wonderful manner the hard 

 spiny sea-urchins to the annulose or worm-like creatures of the great 

 homogangliate class. The Asteridce are highly-organized animals, having a 

 coriaceous integument, with projections more or less spiny, and a hard 

 calcareous skeleton composed of several hundred pieces. Their digestive 

 apparatus consists of a muscular oesophagus, capable of great extension, and 

 a stomach, with long coecal appendages stretching into each ray, but no 

 oral orifice. Their circulatory system is very complex, and permeates every 

 part of the body. It is provided with numerous arterial vessels, and, 

 according to Tiedemann, an organ equivalent in its operation to the functions 

 of a heart. 



The Star- Fishes also possess a curious organ, called by the same author 

 a sand canal, which communicates at one end with a roundish calcareous 

 mark, readily seen on the back of the animal, while with the other it 



