110 OF DIGESTION. 



known that they frequently fill the whole cavity of their body 

 with water, through the mouth, the tentacles, and pores 

 upon the sides, and empty it at intervals through the same 

 openings. And thus the aquatic mollusks introduce water 

 into special cavities of the body, or between their tissues, 

 through various openings, while others pump it into their 

 blood vessels, through pores at the surface of their body. 

 This is the case with most fishes. 



226 a. Besides the more conspicuous organs above de- 

 scribed, there are among the lower animals various micro- 

 scopic apparatus for securing their prey. The lassos of polypi 

 have been already mentioned incidentally, (223.) They are 

 minute cells, each containing a thin thread coiled up in its 

 cavity, which may be thrown out by inversion, and extend to 

 a considerable length beyond the sac to which it is at- 

 tached. Such lassos are grouped in clusters upon the ten- 

 tacles, or scattered upon the sides of the Actinia and of 

 most polypi. They occur also in similar clusters upon the 

 tentacles and the disk of jelly-fishes. The nettling sensa- 

 tion produced by the contact of many of these animals is 

 undoubtedly owing to the lasso cells. Upon most of the 

 smaller animals, they act as a sudden, deadly poison. In 

 Echinoderms, such as star-fishes, and sea-urchins, we find 

 other microscopic organs in the form of clasps, placed upon 

 a movable stalk. The clasps, which may open and shut al- 

 ternately, are composed of serrated or hooked branches, 

 generally three in number, closing concentrically upon each 

 other. With these weapons, star-fishes not more than two 

 inches in diameter may seize and retain shrimps of half 

 that length, notwithstanding their efforts to disentangle them- 

 selves. 



