36 ZOOPHYTES. 



Plate X. Fig. 1. Alcyonidium parasiticum investing the Sertularia falcata. The 

 extremity of the latter still free. 



2. Portion of another specimen. 



3. Part of an extremity of the same, enlarged. 



4. Extremity of another specimen. 



5. Hydra of the same, enlarged. 



6. Specimen, dingy white, investing a slender twig. 



7. The same, enlarged. 



8. Yellowish-brown specimen investing the Flustra carbasea. 



9. Higher edge of the same, with hydrse, enlarged. 



§ 3. Alcyonidium mytili. — Mussel Alcyonidium. — Plate XI. — The 

 precise position of this product is far from obvious, and it is placed here 

 provisionally, for I entertain little doubt of its removal to one more appro- 

 priate. If the Flustra were denominated as a superficies of cells con- 

 taining an ascidian hydra, the product might be allied to it, for the poly- 

 parium resembles a woven texture. Possibly it may prove a transition 

 species to the Flustra membranacea, or an approach to the Lepralia. 

 But being unable to identify it with any of these, its position is qualified 

 as provisional. 



I have scarcely seen this zoophyte under any aspect, or anywhere, 

 otherwise than as a thin patch, of indefinite figure and dimensions, invest- 

 ing the surface of shells, and chiefly those of the mussel. 



It appears as a small spot, or spreads over a superficial area of vari- 

 ous extent, until equalling two or three inches, according to the specimen. 

 The diffusing edge is always curvilinear, the patch of dingy white, and sel- 

 dom thicker than writing paper. — Plate XI. figs. 1, 2, 3, 4. 



The upper surface is soft, wholly composed of numerous cells, appa- 

 rently with an elliptical orifice ; and the lower or deeper part polyangular ; 

 figs. 5, 6. However, the exact form of the adult is not to be easily disco- 

 vered ; and, in general, the real figure of the cell seemed to me to be hexa- 

 gonal. 



During retreat of the hydra?, the surface of a specimen is covered by 

 obtuse prominences, corresponding to those of various other zoophytes oc- 

 cupied by ascidian tenants. 



