OBSERVATIONS ON POLYZOA. 151 



of the muscular and nervous systems are all more or less 

 foreshadowed by the condition of these systems among 

 the higher Polyzoa. 



Hancock, in his admirable memoir on the structure of 

 the Brachiopoda, has described with great clearness the 

 different steps in the complication of the arms. Begin- 

 ning with their simplest aspect in Fredericella, and tracing 

 them to Plumatella, he shows, that, if the tentacles of 

 the latter were approximated, forming a double line on 

 one side, and the arms elongated and twisted spirally, 

 they would be essentially like those of the spiral armed 

 Brachiopods.* In the same memoir he also homologizes 

 the retractor and opercular muscles of Paludicella with 

 the adjustor muscles of the Brachiopod on account of their 

 similar functions, and the parietal band of the former 

 genus with the parietal muscles of Lingula. 



Although I regard functional resemblances as having 

 but little weight in determining homologies, especially 

 where organs, as in the case of the muscles, necessarily 

 change their special office with every change in the posi- 

 tion of the bases of attachment, I, nevertheless, agree 

 with Mr. Hancock in his homologies with the exception of 

 the retractors. These, as I shall presently endeavor to 

 show, are the homologues of the occlusor and divaricator 

 muscles of the Brachiopod. 



Huxley, in his comprehensive article on the Molluscan 

 Archetype, compares an Avicularian with a Brachiopod, 

 remarking the sameness in the position, proportions, and 

 articulations of the valves, and of the divaricator and oc- 

 clusor muscles in both. He also agrees with Mr. Han- 

 cock in regarding the arms of the Polyzooid and Brachio- 

 pod as identical, but shows that they grow in opposite di- 

 rections.! This opposition is better understood if we 

 begin our comparison by imagining the Terebratula to be, 

 as it really is, comparable with a permanently invaginated 

 Polyzooid, and endeavor to modify an invaginated Pluma- 

 tella accordingly. For this purpose the lophophore of 



* Hancock, BracMopoda. Phil. Trans. 1858, also Ann. and Mag. 

 Nat. Hist. 1850, p. 198. 



t Huxley. Encyclopedia, Art. Mollusca. 



