174 JMr. A. Hancock on the Anatomy of the 



be done. Indeed so complete are the results of this naturalist, 

 that, perhaps, the publication of my own may appear almost un- 

 necessary. My investigations, however, carried on as they have 

 been perfectly independent of the researches of others, may not 

 be without some value even where novelty is wanting. Micro* 

 scopic investigations conducted by the aid of transmitted light 

 are liable to error. Frequent examinations in such cases are 

 therefore necessary, and observations independently prosecuted 

 are of peculiar value. Consequently I do not hesitate to give the 

 result of my own labours on this subject, fraught as it is with 

 difficulty, not fearing to mislead in a path already so well trodden. 



Of the anatomy of these animals I shall have to confine myself 

 almost entirely to that of Plumatella, Fredericella and Paludicella. 

 Of Alcyonella I can say but little, having seen only an imperfect 

 specimen, and none of the other freshwater forms have come 

 under my notice. 



Plumatella and Fredericella resemble each other very closely in 

 their anatomical structure, notwithstanding the external differ- 

 ence of their polypes. Paludicella however shows some very in- 

 teresting modifications, particularly in the muscular system : but 

 before entering on the internal anatomy it will be necessary to 

 examine the characters of the polypidom, and to trace its rela- 

 tionship to the polype. 



The polypidom of Plumatella Allmani, PL V. figs. 3, 4 & 5, 

 and of Fredericella sultana is tubular, branched and carinated on 

 the upper surface ; the walls opake, tough and membranous, in- 

 clining to horny. Those of the latter, when examined through 

 the microscope, exhibit a sort of dendritic structure ; the divisions 

 or branches passing in an irregular spiral direction round the 

 tube, are flattened, and extensively anastomosing form for the 

 most part a dense tissue, nowhere more open than just to display 

 the branched character. The walls of Plumatella do not in the 

 least exhibit this structure. In Paludicella the polypidom, fig. 2, 

 is likewise branched and tubular, but not carinated ; it is mem- 

 branous or horny, and becomes enlarged and contracted at cer- 

 tain intervals, dividing the whole, as it were, into cells or com- 

 partments, the external surface being smooth and very glossy. 



All these genera have the polypidom lined with a delicate 

 membrane — the tunic, PI. III. figs. 4 b, b & 5 k, and PL IV. 

 fig. 1 b, which is attached only at certain points to the inner sur- 

 face of the external tube or cell-wall. This in Plumatella and 

 Fredericella becomes excessively delicate towards the orifice, where 

 it apparently blends with the tunic. But in Paludicella the union 

 at this point of the horny wall and tunic cannot be mistaken, 

 though the blending is so gradual that it is impossible to say 

 where one ends and the other begins. And when this polype is 



