DIGESTIVE SYSTEM. 17 



being dilated or contracted, or even completely closed. The intestine is very wide at its 

 origin, and passes up along the side of the cardiac cavity and oesophagus, rapidly diminishing 

 in diameter till it terminates in a distinct anus by perforating the tentacular sheath just below 

 the mouth, and at the concave side of the lophophore. 



b. Gymnolcemata. 



In FaludiccUa articulata, the mouth (PI. X, fig. 5) is a perfectly circular orifice, with 

 slightly projectile margin, and is totally destitute of the valve-like appendage which, unless 

 UrnaieUa should prove an exception, is found in all the other fresh-water species. The 

 upper part of the oesophagus (fig. 3,/) is wide, and may perhaps here, more decidedly than 

 in the other species, be distinguished as jiliari/nx. It soon contracts into a long narrow tube, 

 which leads to an oval sac (y) corresponding to the cardiac cavity of the stomach in the 

 other fresh-water Polyzoa, and to the gizzard in certain marine species. This sac is much 

 more distinct from the great cavity of the stomach than in the other Polyzoa of fresh water. 

 It enters this cavity near its upper extremity, and presents here a well-marked constriction ; 

 in extreme retraction of the polypide it is bent back upon the rest of the stomach. The great 

 cavity (/) of the stomach is of a nearly cylindrical figure ; from its upper extremity arises the 

 intestine (//). This tube presents, just after its origin, a wide dilation, and then suddenly con- 

 tracting, continues as a narrow cylindrical canal to its termination just below the mouth. 



The Idstolof/kal sfrudure of the alimentary tube in both the phylactoltematous and 

 gymnolsematous forms is somewhat complex. It may, however, be easily enough made out 

 in the different genera. Wherever I have had a good opportunity of examining it, I have 

 succeeded in detecting in the stomach three distinct layers. Internally is a yellowish-brown 

 layer (PI. Ill, fig. 7 k), which is thrown into large longitudinal rugae. These rugae become less 

 decided towards the fundus of the stomach, and in Paludicella they are entirely absent. This 

 internal layer is composed of easily separable spherical cells (PI. II, figs. 6, 7), containing a 

 colourless fluid, in which floats a secondary cell, with yellowish-brown contents. When the 

 animal has been left long without food, the brown matter disappears from the cells, and the 

 stomach becomes colourless. The inner layer of the stomach is thus distinctly glandular, 

 and may be fairly viewed as the representative of a liver, the cells with brown contents 

 being manifestly true secreting cells, destined for the elaboration of the bile, and capable of 

 being set free by the rupture of the cell which encloses them. 



This layer passes externally into a more compact layer (PL III, fig. 7, X) composed of 

 smaller, simple cells (PI. II, fig. 8), with colourless contents and a brilliant nucleus. 



The third or most external layer is a thin membrane (PI. Ill, fig. 7, /*) ; it possesses also 

 an undoubted cellular structure; it admits of being traced uninterruptedly over the whole tract 

 of the alimentary canal, and, on the application of acetic acid, becomes frequently raised from 

 the subjacent layer. Delicate circular striae may generally be distinctly observed in it, they may 

 be seen surrounding the stomach, and are probably muscular fibres ; they are particularly 

 evident in Alcyonella and Plumatella towards the fundus of the stomach ; they become less 



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