DIGESTIVE SYSTEM. 15 



itself is imperfect, admitting of a communication through its centre between two neighbouring 

 cells. In Alci/onella fitngosa, and in Fredericella sultana, imperfect septa may here and there 

 be observed, while Cristatella and Lophopus would seem to be quite deprived of them, the cells 

 in these genera all opening into one another. 



The ectocyst of the fresh-water Polyzoa appears in every case to be absolutely structure- 

 less. The fact of cellulose being a constituent of the test of the Tunicata, induced me to look 

 for it in the ectocyst of the Polyzoa, but I have never succeeded in obtaining satisfactory 

 evidence of its existence in any of the tissues of the Polyzoa, either fresh-water or marine. 

 I observed, however, that in one instance the ectocyst of Plumatella rcpens, after lying for 

 several weeks in a concentrated solution of caustic potash, in which it had been first boiled, 

 presented under the microscope, at one or two points, a distinctly blue tint on being wetted 

 with tincture of iodine, and then with sulphuric acid, a fact which would seem to point to the 

 possibility of the true cellulose reaction being only masked by the presence of other consti- 

 tuents, as we know to be the case in some of the tissues of plants which fail to strike a blue 

 colour with iodine and sulphuric acid, until the removal, by a somewhat tedious process, of the 

 adventitious matter. I do not, however, lay any stress on the blue colour produced in the 

 above instance, as it did not occur in others, and was therefore probably accidental. Upon 

 the whole, the reactions of the pergamentaceous ectocysts of Phuiiafe/la, &c., are rather 

 in favour of this tissue being composed of chitine. It is quite insoluble in strong acetic 

 acid, and in a concentrated solution of caustic potash, even when exposed to prolonged 

 boiling in these fluids, or after month-long maceration in them ; but it is soluble in concen- 

 trated nitric, hydrochloric, and sulphuric acid. Successive boiling in water, alcohol, ether, 

 acetic acid, and caustic potash, renders it nearly colourless, without in .any way altering its 

 form, all which properties are among the essential characteristics of chitine, which would 

 thus seem to replace in the entocyst of the Polyzoa the cellulose of the test of the Tunicata, 

 unless more elaborately conducted researches shall prove the essential constituent of the 

 tissue in question be identical with the more highly nitrogenized conchiolin of ordinary 

 Mollusca.* 



(2.) Organs of Digestion. 



The digestive system is very similar in all those species in which the lophophore is 

 bilateral {Pfigjactolmnata) ; these we shall therefore consider together ; PaludiceUa, w'hich, 

 with the exception of Urnatella (?), is the only fresh-water representative of the division with 

 orbicular lophophore {Ggnmokemata). presents some peculiarities, and should be examined by 

 itself. 



* See Sclilossberger, Zur naheren Kenntniss der Mushelsclialen, des Byssus uud der Chitinfrage 

 ' Ann. der Chem. uud Pharm.,' xcviii, 99. 



