OBSERVATIONS ON POLYZOA. 101 



may incline the polypide to the right or left, by reciprocal 

 contraction and expansion on the opposite sides of the 

 body, or sway it towards the abdominal or dorsal sides. 



In Fredericella, and the Plumatellidse it is necessary 

 for both the cesophagal and brachial retractors to act recip- 

 rocally in order to bring about the dorsal or abdominal flex- 

 ures of the polypide ; but in Pectinatella and Cristatella 

 the brachial retractors are probably sufficient by themselves. 

 The diifusion of the filaments of the brachial retractors is 

 so great in these genera, that those on the abdominal or 

 dorsal sides would have sufficient leverage and be fully 

 competent to bend the polypide either forwards or back- 

 wards. 



The muscles of the invagiuated fold are in two sets. 

 These have been previously described by Prof. Allman 

 under the names of Posterior and Anterior Parieto-vaginal 

 muscles ; but, since I differ from him in the location of the 

 poles of the body, I have thought it advisable to alter these 

 names in order to avoid confusion. The first set or Ante- 

 rior Retentors, equivalent to the posterior parieto-vaginals 

 of Allman, consists of a single annular row attached to the 

 lower edo;e of the invaginated fold (PI. 7, figs. 4, 5, PL 

 8, figs. 2, 5, PL 9, fig. 12, PL 10, fig. 1, PL 13, fig. 5, 

 PL 14, fig. 1, N). 



The second set, or Posterior Retentors, equivalent to 

 the anterior parieto-vaginal muscles of Allman, consists of 

 smaller muscles attached to the side of the invaginated fold 

 (PL 7, fig. 4, PL 8, figs. 2, 5, Ph. 10, 12, fig. 1, N'). 



The latter are not, as has been hitherto supposed, irreg- 

 ularly disposed, but are arranged in rows, the bundles being 

 placed alternately in each row. They assist the anterior 

 retentors to retain the invaginated fold, and sustain the 

 tentacular sheath in the centre of the ccenoecium, when the 

 polypide is retracted. Deep external furrows are often 

 caused in the coeno3cium by these posterior retractors, 

 while the polypide is fully expanded (PL 8, fig. 5, N'). 

 Similar folds were, also, noticed in the polypidal en- 

 docyst of Pectinatella occasioned by a few fibres, prob- 

 ably abnormal, which connected the oesophagus and oppo- 



