163 

 TtiNICATA, 



Fig. 154. POROPHORA.* 



THERE arc some Mollusks which are not naked like the slug, 

 nor provided with a shelly citadel like the oyster, but are 

 furnished with a kind of leathery covering or tunic, and aro 

 hence termed " TunicatecL" They have already been casually 

 mentioned in our notice of the higher organized polypes 

 (page 27), to which, in certain points of structure, they pre- 

 sent a considerable affinity. Some of them are aggregated 

 together, and form compound animals ; others are solitary, 

 and so inert that to common observers they exhibit no indi- 

 cations of life. The kind best known to our fishermen is a 

 solitary species (Ascidia communis) about the size of the 

 largest common mussel, and to which, from its shape, the 

 name of "paps" is given. The exterior is darkish, warty, and 

 unattractive, and exhibits two orifices, from one of which the 

 animal can squirt water with considerable force. The internal 

 structure is extremely beautiful and delicate. A great part 

 of it consists of a large chamber, lined with a delicate mem- 

 brane, over which the blood-vessels are widely distributed. 

 The surface is abundantly covered with vibratile cilia; and, 

 as the sea-water is freely admitted into the cavity, the cease- 

 less action of the cilia propels it in currents over the surface 

 of the membrane, which thus performs the office of an internal 

 gill. The chamber itself is hence appropriately termed the 

 " branchial sac." Through it the nourishment of the animal 

 must pass ere it can be received into the stomach, which is at 



*Fig. 154. m, Mouth. *, Stomach. f, Intestine. o, Orifice /, Common 

 Stem. The arrows indicate the direction of the currents of water subservient to 

 respiration. 



