Organs of Invertebrate Animals. 45 



kind or another arranged around the entrance to the 

 pharynx. Lateral jaws are not to be met with here : 

 and even the teeth are not always teeth, for, as in the 

 case of the sand-lug, the teeth become external spines, 

 which can be used as organs of prehension, when the 

 proboscis is everted. Around the mouth, also, and 

 coming under the head of oral appendages, may be 

 certain labial tentacles and branchial processes, but these 

 do not require special notice now, for all that may be 

 said about them is included in what has to be said 

 immediately upon the oral appendages of the radiata. 



Among the radiata the bilateral arrangement of 

 limbs has given place to the radiate, and all the limbs 

 may be looked upon as oral appendages. So it is in the 

 great class of polypes : so also in the star-fishes and the 

 cephalopods. In each of these cases the polype-type 

 prevails, and, except in different creatures, the different 

 limbs, with few exceptions, are all alike. The case is 

 one, indeed, in which, in this similarity in the limbs, it is 

 possible to see that the mandibles and maxillae of the 

 mandibulate or haustellate mouth may be nothing more 

 than modifications of a certain number of the so-called 

 arms of a radiate animal, and that the upper and lower 

 lips, as well as the tongue and other parts of the same 

 mouth, may in like manner be substantially other arms 

 of the same sort. Nay, it is possible to go further in the 

 same direction and find much in favour of a still wider 

 generalization. In many of the star-fishes the roughened 

 bases of the rays are made to do the work of the special 

 teeth of the sea-urchin, and it is difficult not to see that 

 these teeth are modifications of the same organs which 

 exist on the ray of the star-fish as mere calcareous 

 tubercles, and which are developed in the sea-urchin 

 into long and strong articulated spines. Organs which 



