MANUAL OF ZOOLOGY. 



eight long radiating processes, which give it a stellate appear- 

 ance ; and the branchial sac is so flattened as to become 

 nearly horizontal. 



No Tunicates are known with certainty to have been pre- 

 served in the fossil condition, but the singular Silurian genus 

 Pasceolus has been doubtfully referred to this class. 



CHAPTER XLIII. 

 BRACHIOPODA. 



CLASS III. BRACHIOPODA (Palliobranchiata}. The members 

 of this class are defined by the possession of a body protected 

 by a bivalve shell, which is lined by an expansion of the integ- 

 ument, or " mantle" The mouth is furnished with two long 

 cirriferous arms. The nervous system consists of a single gan- 

 glion, placed in the re-entering angle between the gullet and the 

 rectum, so that the intestine has a " neural flexure" 



The Brachiopoda are essentially very similar in structure to 

 the Polyzoa, from which they are distinguished by the fact that 

 they are never composite, and by the possession of a bivalve, 

 calcareous, or sub-calcareous shell. They are commonly known 

 as " Lamp-shells," and are all inhabitants of the sea. All the 

 living forms, except Lingula pyramidata, are fixed to some 

 solid object in their adult condition ; but there is good reason 

 to believe that many of the fossil forms were unattached and 

 free in their fully grown condition. From the presence of a 

 bivalve shell, the Brachiopods have often been placed near the 

 true bivalve Mollusca (the Lamellibranchiatd), but their organi- 

 sation is very much inferior, and there are also sufficient dif- 

 ferences in the shell to justify their separation. 



The two valves of the shell of any Brachiopod (figs. 202, 

 204) are articulated together by an apparatus ^f^teeth and 

 sockets, or are kept in apposition by muscular action alone. 

 One of the valves is always slightly, sometimes greatly, larger 

 than the other, so that the shell is said to be " inequi valve." 

 As regards the contained animal, the position of Jhe valves is 

 anterior and posterior, so that they^r^lrTerefofe termed re- 

 spectively the " ventral " and " dorsal " valves. In the ordi- 

 nary bivalve Mollusca (Lamellibranchiata), on the other hand, 



