Xll 



INTRODUCTION. 



its gonophore. It is difficult to imagine what relation 

 these can bear to the economy of the animal. 



Fig. ii. Fig. iii. 



Groups of these bean-shaped cells are also present on 

 many of the tentacles of the latter zoophyte, a little above 

 the base, and form a beautiful ring of prominent pearly 

 bosses. 



Besides the thread-cells, the arm of the Hydroid bears 

 another organ, which has been named by Dr. Wright the 

 palpocil, and which is connected, no doubt, with the sense 

 of touch. It consists of a long and delicate spine, springing 

 from a small bulb, which is buried in the ectoderm. These 

 palpocils or sensitive hairs are scattered over the tentacles 

 in many species, and over other portions of the body, 

 and must aid the capture of prey by giving instant notice 

 of the presence of any animalcule or other small creature 

 that may brush against them. It may be their function to 

 rouse the thread-cells into action. 



