198 Mr. A. Hancock on the Anatomy of the 



led Professor MiJne-Edwards, in his valuable memoir on the 

 1 Ascidiens Composees/ to propose the separation of the Tunicata 

 from the Mollusca, and the formation of them into a distinct 

 group to be placed between the bivalves and the polypes. 



We then cannot find a passage from the Bryozoa through 

 the Ascidiat into the Lamellibranchiata. There are however two 

 distinct branches of the Ascidian polypes, — one with the tentacles 

 arranged in a circle about the mouth, — the other having them 

 supported on two lateral oral arms. The former of these branches 

 passes into the Ascidian, — the latter I shall endeavour to show 

 is connected with the Brachiopoda. 



This connexion is at once suggested by the resemblance that 

 exists between the oral arms of Plumatella and Alcyonella and 

 the characteristic brachial organs in the Brachiopoda, parti- 

 cularly of those in Lingula. In both the Brachiopoda and the 

 Bryozoa, the arms rise from the sides of the mouth in the same 

 manner, the bases of the arms being confluent ; and the tentacles 

 or cirrhi forming a continuous series. In both the arms are 

 hollow, and the tentacles and cirrhi are tough and non-contractile ; 

 and in both they are prehensile organs after the same fashion. 

 The digestive organs of both are very similar ; and the whole of 

 the Brachiopoda are fixed, and so are the Bryozoa, with but one 

 exception. 



But what is still more remarkable, the muscular systems of 

 both are arranged much in the same manner, particularly as 

 respects Terebratula and Paludicella w r ith most of its marine 

 congeners. In Terebratula, as the animal is fixed within the 

 shell, of course there can be nothing resembling the polype 

 retractors ; but the shell muscles of Terebratula will be found to 

 work exactly on the same principle as those provided to draw in 

 the margins of the cell-orifice in Paludicella and Bowerbankia, 

 and called by Dr. Farre opercular muscles. 



There are four sets of muscles in connexion with the shell in 

 Terebratula chilensis as dissected by Owen, two from each valve ; 

 and they all pass diagonally downwards, and with one exception 

 go to be inserted in the pedicle ; so that when they contract the 

 valves will be closed. These muscles then have in fact their 

 origin in the pedicle as stated by Owen, and acting from thence 

 upon the moveable points of their insertions, operate precisely 

 in the same way as the tube-retractors of the polypes last men- 

 tioned do on the lips of the orifice. The action is the same in 

 both ; and were the cell-walls of Bowerbankia, for instance, cal- 

 cified and divided longitudinally into two portions or valves, they 

 would be made to close just as the valves do in Terebratula. 

 The set of muscles alluded to as not passing into the pedicle 

 comes from the perforate valve, and inclining downwards is 



