3 68 



LOWER FORMS OF MARINE LIFE 



the sea-squirts do not adopt this sedentary mode of life, but are permanently free- 

 swimming. Typically the sedentary adults are in the form of leathery or 

 cartilaginous bags, tixed by the lower end to rocks or the sea-bottom, and with one 

 aperture at the summit and a second on a protuberance about one-third down one 

 side ; the upper aperture being for the intake of sea-water, and the lower one for 

 its exit. There are, however, various departures from this type, some kinds forming 

 tleshy coatings on seaweeds or rocks, others solid flesh}? masses, while yet others 



f*<^? 



A SIMPLE SEA -SQUIRT. 



may be free-swimming and barrel-shaped, in some instances united together into 

 chains or hollow cylinders. The ordinary Asciclia mentula, which may be taken 

 as a very typical representative of the class, lives on muddy bottoms, and 

 measures about four inches from base to summit. At low tide, when un- 

 disturbed, both apertures of the outer bag are open and maintain a constant 

 current of water through the interior of the organism ; but when disturbed they 

 are suddenly closed, jets of water being squirted out as the closure takes place. 

 With regard to the internal structure of these strange organisms, it may be 

 mentioned that the upper part of the gullet, or pharynx, is dilated into a huge 



