B. IT.] THE HISTORY OF ANIMALS. 87 



other membrane, are the ova, the same number in all, they 

 are five in number, and uneven. 



6. The black substance is joined above to the origin of the 

 teeth, this black substance is bitter and not eatable ; in many 

 animals there is either this substance or its analogue, for it is 

 found in tortoises, toads, frogs, turbinated shells, and in the 

 malacia ; these parts differ in colour, but are entirely or nearly 

 uneatable. The body of the echinus is undivided from be- 

 ginning to end, but the shell is not so when seen through, 

 for it is like a lantern, with no skin around it. The echinus 

 uses its spines as feet, for it moves along by leaning upon 

 them and moving them. 



CHAPTER VI. 



1. THE creatures called tethya 1 have a most distinct charac- 

 ter, for in these alone is the whole body concealed in a 

 shell. Their shell is intermediate between skin and shell, 

 so that it can be cut like hard leather : this shell-like sub- 

 stance is attached to rocks ; in it there are two perforations, 

 quite distant from each other, and not easily seen, by which 

 it excludes and receives water, for it has no visible excre- 

 ment as other testacea, neither like the echinus, nor the 

 substance called mecon. 



2. When laid open, there is first of all a sinewy membrane 

 lining the shell-like substance, within this the fleshy sub- 

 stance of the tethyon; Unlike any other creature, its flesh, 

 however, is alike throughout, and it is united in two places 

 to the membrane and the skin from the side, and at its 

 points of union it is narrower on each side ; by these places 

 it reaches to the external perforations which pass through 

 the shell ; there it both parts with and receives food and 

 moisture, as if one were the mouth, the other the anus, the 

 one is thick, the other thinner. 



3. Internally there is a cavity at each end, and a passage 

 passes through it ; there is a fluid in both the cavities. Be- 

 sides this, it has no sensitive or organic member, nor is there 

 any excrementitious matter, as I said before. The colour of 

 the tethyon is partly ochreous, partly red. 



4. The class acalephe 2 is peculiar ; it adheres to rocks like 

 gome of the testacea, but at times it is washed off. It is not 



1 Ascidian molluske. 2 Actiniae. 



