OF THE EOTATOHIA. 417 



Leydig represents this tube to be lined by a continuation of the chitinous 

 inner layer of the maxillary bulb, and uses this view to explain the distinct- 

 ness of outline frequently remarkable in the walls when of considerable 

 thickness, e. g. in Notommata centrura (XXXYIII. 26 q) and N. tardigrada. 

 This sharj) contour is especially manifest dimng contraction of the canal, 

 whereby it is thrown into transverse folds or wrinkles, noticed by Ehrenberg 

 under the title of " hard oesophageal folds," and elsewhere of a " rather firm 

 framework at the commencement of the oesophagus." Leydig adds — " The 

 organs described by Ehrenberg in Notommata saccigera, as ' large vibrating 

 gills,' must, I think, be considered transverse folds of the chitin membrane 

 in question." The existence of so dense a lining to the gastric tube implies 

 the absence of cilia on its surface ; and, in fact, Leydig declares he has never 

 seen the least sign of such organs, although both Perty and Williamson affirm 

 their existence. The folds into which this tube is thrown when contracted 

 are occasionally {e. g. in Notommata Sieboldii) (XXXVII. 22) longitudinal 

 instead of transverse. Mr. Gosse says of it that it is " composed of longitu- 

 dinal and annular contractile tissue," and that, at least in Asplanchna prio- 

 donta (XXXYI. 9 1), " it is capable of immense dilatation, but commonly takes 

 the form of a slender tube Avith the lower extremity swoUen, where an oval 

 pancreatic gland is attached on each side. The passage of a small morsel, 

 such as a Chilomonas, shows that the walls of this organ are thick, leaving 

 only a slender tube when corrugated." However, in different species the 

 width and the thickness of its waRs vary much. The proventricular canal 

 has a considerable length in Diglena and Sgnchceta ; it is rather long in 

 Triarthra, Lacimdaria, and Hydatina, and very short in Euclilunidota, Bra- 

 chioncm, and Melicerta. In not a few genera it is altogether wanting, the 

 maxillary bulb being superposed immediately upon the stomach : such are 

 Ascomorpha and the genera of the family Philodincea. 



The stomach succeeds to the gastric canal as a distinct segment separated 

 from the alimentary tube below by a constriction, and is remarkable also in 

 general by its greater capacity (XL. 1 e). Leydig affirms that a portion of 

 the digestive canal separated from the rest by a constriction, and essentially 

 representing a stomach, exists in aU true Rotatoria; but other writers describe, 

 as in Philodina (XXXYIII. 1, 2), and in Lindia (XL. 1, 3), the existence of 

 a straight, slender, fLmnel-Like alimentary canal extending from the mouth 

 to the cloaca without any constriction or any stomach dilatation. In Hy- 

 dat'ina and Synchceta, Perty says the canal is uniform in calibre, without any 

 stomach-Hke expansion ; yet Cohn distinguishes the narrower lower end of 

 the alimentary tube of Hydatina as an intestine, because it is less constantly 

 occupied with food, is colouiiess, and, unlike the stomach, has no such cells 

 on its waU. Moreover, as an irregularity, he twice met with a sphincter- 

 hke constriction {pylorus) separating the two. In Euchlanis and Brachionus, 

 on the other hand, the division is clearly indicated (XXXIX. 16). 



The opposing statements of authors on this question may probably be re- 

 conciled on the supposition that, of different observers, some have viewed the 

 canal when it has been fiiU and distended, others when empty and contracted, 

 and that the constriction indicating a definite stomach has appeared only 

 during repletion, just as happens with the human stomach, which, when full 

 and engaged ia digestion, is deeply constricted, and for the time appears 

 almost like a double organ. 



Ehrenberg distinguished four types of Rotatoria, according to the cha- 

 racters of the alimentary tube, which he respectively named — 1. Trachelo- 

 gastnca ; 2. Coelogastrica ; 3. Gasterodela ; 4. Trachelocystica. 



1. The Tracheloqastrica comprehended animals having a long filiform gullet, 



2e 



