Chap. IV.] ECHINODERMS. 121 



oesophagus are ordinarily strengthened by the deposit 

 of calcareous plates, which are sometimes very regu- 

 larly arranged. 



The walls of the intestines of Echinoderms are hi 

 all cases remarkably thin, and but feebly provided with 

 muscular tissue, a somewhat remarkable arrangement, 

 when we reflect that the movement of food in their 

 digestive tract can be by no means aided by the 

 pressure of their body walls on the enteric tube 

 within. 



In the Crinoidea the anal is always near the oral 

 orifice, and is placed on a projecting cone ; in Holopus, 

 as in some starfishes (e.g. Astropecten) and in all 

 Ophiuroids, the anus is lost, so that here we have an 

 example of the fact that the absence of an anus is not 

 always to be regarded as a primitive condition. 

 There can be no reasonable doubt that the Crinoids 

 are older than the rest of the Echinoderms, and it is 

 only in the most aberrant of these that we find an 

 anus absent. Where an anus is present it is, except 

 in Crinoids, placed typically at the opposite pole of the 

 body to the mouth ; but in the irregular Echinids we 

 find a most interesting series in the way of modifica- 

 tion : thus, in Rhyncopygus it is on the " back," but 

 not at the apical pole ; in Echinolampas it is at the 

 edge of the test, where the upper passes into the under 

 ^urface ; while in Echinoneus it is quite close to the 

 mouth, and, therefore, completely on the ventral 

 surface. 



The intestine is either saccular, as in the aproctous 

 Echinoderms, or spirally coiled as in Crinoids and 

 developing starfishes, or looped as in Holothurians ; 

 in the proctuchous Asteroids and in some Echinids 

 it is provided with caeca, which in the former are 

 paired, and extend some way down the cavity of each 

 of the arms. These so-called " hepatic caeca " have 

 been found to have on fibrin the action of peptic 



