OBSERVATIONS ON POLYZOA. 155 



and Khynclioiiella psittacea may be looked upoii as rela- 

 tively outside of the divaricators. 



The similarity between these three sets and the 

 retractors and retentors of a Plumatella may be readily 

 shown, if we bear in mind that the invaginated specimen 

 compared on page 151 with a Brachiopod was rotated 

 towards the dorsal side. If we suppose that only the 

 muscles attached to the lophophore, the retractors, par- 

 took of its rotation, it is evident, that the posterior 

 lophophoric extremities of the retractors became dorsal, 

 and the anterior ventral, while the retentors were left 

 comparatively unchanged, running in the direction of the 

 antero-posterior axis. 



The occlusors of Terebratula are in two distinct pairs 

 on the ventral side, but each lateral pair blends into 

 one stock and inclines inwards as it passes dorsally, and 

 they have the alimentary canal between them. The gastric 

 and oesophagal retractors of Plumatella make up two 

 distinct pairs more or less intimately connected through- 

 out, but widely separated at the anterior end, and inclin- 

 ing towards each other as they pass posteriorly ; they 

 also have the alimentary canal between them. 



The divaricators of Terebratula are an independent 

 pair next to the occlusors, spread out, however, in allied 

 forms into two pairs on the dorsal side. 



The brachial retractors of Plumatella are also an 

 independent pair, spread out into numerous fibres at the 

 posterior end, and closely gathered together below, and 

 are next to the oesophageal. 



The adjustors are in four pairs, arranged in nearly 

 parallel bands outside of all the other muscles, and run 

 towards the posterior end. 



The retentors of Plumatella are arranged in nearlv 

 parallel bands, but are not separated into pairs because 

 the coenoecial orifice is not widened laterally, and, there- 

 fore, does not divide them into a dorsal and ventral set 

 as in Terebratula. They are also outside of all the 

 other muscles, and run towards the posterior end. 



Mr. Hancock, as previously remarked,* also homolo- 



* Hancock, Ann. and Mag. Nat. Hist., 1850. 



