34 HYDROZOA. 



to Huxley and Kolliker, is the structure of the 

 tentacular branches in Forskalia, and which, 

 under slight modifications, repeats itself in many- 

 other forms of Physophoridce and Calycophoiidw, 

 Within the sacculus of the last-mentioned order, 

 occurs a peculiar zig-zag muscular cord, known as 

 the ^' angel-band," the nature of which is but im- 

 perfectly known. 



Where the pedicle and sacculus unite, a solid 

 process of the ectoderm has been observed to 

 originate in some Physophoridce. From its in- 

 vesting the sacculus in the form of a hood, this 

 orofan has received the name of ^involucrum.' 



The mode of action of the tentacles, as appen- 

 dages for prehension, has been sufficiently explained 

 in our account of the Hydra. 



The name of ' nematophores ' has been given by 

 Mr. Busk to peculiar csecal processes, distinct from 

 the oral tentacles, which are found on the coenosarc 

 of some Sertularidw. Like the rest of the coenosarc 

 these processes are invested with a stiff, horny, 

 layer, open at the distal end of the nematophore, 

 beneath which are embedded many large thread- 

 cells. The nematophores probably serve as organs 

 of offence. They are most numerous in the genus 

 Plumularia. 



5. Te$cunieiitary Organs. — The tegumentary 

 system in the Hydrozoa is composed wholly of 

 the, in general, ciliated, ectoderm, and the rich 

 supply of thread-cells to which this layer gives 

 rise. Usually it appears more or less vacuolated, 

 or it may even become changed into a gelatinous 

 mass. The thickened disc of the Medusida' and 

 TAicernaridcv, in some structureless or but faintly 



