29 



III. ASCIDIAD^. 



Rarely is the dredge drawn up from any sea-bed at all 

 prolific in sub-marine creatures, without containing few or 

 many irregularly shaped leathery bodies, fixed to sea-weed, 

 rock, or shell by one extremity or by one side, free at 

 the other, and presenting two more or less prominent ori- 

 fices, from which on the slightest pressure the sea-water is 

 ejected with great force. On the sea-sliore, when the tide 

 is out, we find similar bodies attached to the under 

 surface of rough stones. They are variously, often splen- 

 didly coloured, but otherwise are unattractive or even re- 

 pulsive in aspect. These creatures are Ascidia, properly so 

 called. Numbers of them are often found clustering among 

 tangles, like bunches of some strange semi-transparent fruit. 

 They are very apathetic and inactive, living upon micro- 

 scopic creatures drawn in with currents of water by means 

 of their ciliated respiratory organs. 



The leathery case is often encrusted with stones and 

 shells, decorated with parasitical though ornamental plumes 

 of corallines, and not seldom perforated by bivalves, which 

 lodge themselves snugly in the tough but smooth skin; it is 

 the analogue of the true shell of conchiferous Mollusca. It 

 is a sac, closed except at two orifices, one of which is 

 branchial, the other anal. This elastic gelatinous or coria- 

 ceous envelope is called the test, and encloses a second tu- 

 nic or mantle, which is muscular and adheres to the first 

 only near the orifices. The branchial sac lines the interior 

 of the mantle in part. It is both respiratory and pharyn- 

 geal. The remainder of the cavity is occupied with the. 

 principal organs of digestion, circulation, and generation. 

 The chief nervous centre is situated between the two 

 openings of the muscular tunic. The sexes of Ascidiee are 

 distinct. 



