Organs of Invertebrate Animals. 47 



should be perforated by a poison duct, for this may be 

 no more than a modification of the hollow tentacle which 

 is natural to the bryozoic and anthozoic polypes. It is 

 possible that the close connexion between the gill and 

 the foot of which so many instances are met with in the 

 articulata may explain why it is that the gill and the 

 " foot " are related to each other in the same manner in 

 molluscs, and why, for the same reason, it becomes 

 necessary to regard the latter foot as a modification of 

 an oral appendage rather than as a vague " development 

 from the ventral surface." It is a matter of indifference, 

 however, whether the foot In question be an oral appen- 

 dage or not. It is radically similar, and that is all that 

 need be said about it, and also about the two arms of 

 the branchiopods, and the two wings of the pteropods 

 which have some claim to be regarded in the same light 

 except this, that these latter organs lead back from 

 the radiate arrangement of appendages to that bilateral 

 arrangement which is natural to the crustacean and to 

 the higher forms of articulate creatures. 



5. The eyes of the prawn are organs of a very 

 different sort to those which have been under considera- 

 tion, but the peculiarities are not so very real as they 

 seem to be at first sight. They are compound eyes, 

 that is, they are composed of many eyelets ; they are 

 two in number, arranged bilaterally ; and they are 

 placed upon peduncles made moveable by being jointed 

 at their base. In the Limulus, and in certain other 

 entomostracan crustaceans, several simple eyes (ocelli 

 or stemmata) are associated with compound eyes, pedun- 

 culated or sessile, each simple eye, like every single 

 eyelet of the compound eye, having its separate lens, 

 and nerve, and pigmentary layer, and humours, as well 

 as a transparent speck of integument which does duty as 



