2/8 COMPARATIVE ZOOLOGY 



stricted at the extremities, digestion takes place ; but the 

 product passes freely into all the surrounding chambers, 

 along with the water for respiration (Fig. 236). The 

 Medusae, or jelly fishes, preserve the same type of a diges- 

 tive apparatus ; but the sac is cut off from the general 

 cavity, and numerous canals radiate from it to a circular 

 canal near the margin of the disk (Fig. 21). In the star- 

 fishes (Fig. 323), we find a great advance. The saclike 

 stomach sends off two glandular branches to each arm, 

 which doubtless furnish a fluid to aid in digestion (so- 

 called hepatic coeca). There is also an anus present in 

 some forms, but it hardly serves to pass off the waste 

 matter. 



Thus far we have seen but one opening to the diges- 

 tive cavity, rejected portions returning by the same 

 road by which they enter. But a true alimentary canal 

 should have an anal aperture distinct from the oral. 

 The simplest form of such a canal is exhibited by the 

 sponge, in its system of absorbent pores for the entrance 

 of liquid, and of several main channels for its discharge. 

 The apparatus, however, is not marked off from the 

 general cavity of the body, and digestion is not distinct 

 from circulation. 90 



The sea urchin presents us with an important advance 

 one cavity with two orifices ; and the complicated 

 apparatus of higher animals is but the development of 

 this type. This alimentary canal begins in a mouth 

 well provided with teeth and muscles, and extends 

 spirally to its outlet, which generally opens on the 

 upper, or opposite, surface. Moreover, while in some 

 of the worms the canal is a simple tube running through 

 the axis of the cylindrical body from oral orifice to anal 

 aperture, the canal of the sea urchin shows a distinction 

 of parts, foreshadowing the pharynx, gullet, stomach, 

 and intestine. Both mouth and vent have muscles for 



