INTRODUCTION. 



IX 



lariidce it takes the latter form, and is a very conspicuous 

 feature. (Woodcut, fig. i. a.) It is continually changing 



Fig. i. 



its shape, now enormously distended, now flattened down 

 and with the lips thrown back, so as to form a saucer-like 

 disk, now opening and closing rapidly, but never long the 

 same. It is an admirable instrument, in conjunction with 

 the tentacles, for the selection and prehension of food. In 

 some Hydroids there is a marked constriction at the base 

 of the proboscis; and in the curious genus Ophiodes 

 (Hincks) the body is divided by a depression a little below 

 the tentacles into two regions, a pharyngeal and gastric. 

 Reichert distinguishes the narrow between the proboscis 

 and the stomach as " the cesophageal passage." In gene- 

 ral, however, the structure of the digestive sac is perfectly 

 simple, and no defined " regions " can be recognized. 



Within the stomachs of the alimentary zooids the food 

 is digested and prepared for the nutrition of the whole 

 structure. The polypites are the feeders of the common- 

 wealth, and the unceasing activity of many thousands of 

 them in the larger species is engaged in keeping up the 

 necessary supplies. 



