ISOMBTRA VIVIPARA. 45 



been decalcified, stained, and mounted in balsam, so as to show the inner 

 anatomy in Mo. Longitudinal sections of corresponding stages are repre- 

 sented in plate xxi, figures 4 and 5, and transverse sections in plate xix, 

 figures 5 to 10, and plate xx, figures 1 and 2 and figures 3 to 8. 



The epithelium of the oral surface has become thin, in marked distinction 

 from the previous stage, before the opening up of the vestibulum (plate xxi, 

 figure 4, comp. with plate xvm, figures 1 and 2). The mouth may protrude 

 above the oral surface like a small funnel, so as to appear in transverse 

 sections as a ring (plate xx, figure 2) . In one case I have found a thickening 

 of the esophageal wall in the anal interradius, which makes a distinctly 

 limited furrow that ultimately closes into a narrow canal. The thickening 

 begins at the level of the hydroccel ring (plate xix, figure 5) and continues 

 some way below the rectum, ending as a small ridge that rises into the 

 lumen of the stomach (plate xix, figures 5 to 10). I am at a loss to explain 

 this structure, which may be some abnormality, perhaps due to the fact 

 that the stomach in this specimen is strongly dilated by the embryo which 

 has been eaten. In any case I have thought it worth while mentioning 

 and giving the series of figures quoted to illustrate it. In plate xx, figure 

 4, is also seen a thickening of the esophageal wall in the anal interradius. 

 In this specimen, however, the thickening soon widens in passing down- 

 wards, and passes gradually into the normaFcondition of the entodermal 

 epithelium. 



The stomach is provided with folds (plate xx, figure 6; plate xxi, figures 

 5 and 6) ; in the strongly dilated stomach of the specimen that has eaten an 

 embryo (plate xix, figure 5) these folds have disappeared; the anal opening 

 has been formed (plate xx, figure 5; plate xxi, figure 6); that the opening is 

 indistinct in plate xix, figure 8, is evidently due to pressure on account of 

 the contents of the stomach. 



The hydrocoel ring is not yet completely closed. In plate xix, figure 5, 

 and plate xx, figure 3, is seen the narrow wall that still separates the two 

 ends of it. Numerous trabeculse have been formed in its lumen. A thicken- 

 ing of the epithelium over the upper side of the hydroccel ring (plate xxi, 

 figure 5 r. n.) is the ring-nerve which has begun to develop. Below the nerve 

 is an indication of circular muscle fibers (comp. Seeliger, op. cit., taf. 21, 

 figure 174). The stone canal has lengthened considerably (plate xxi, figure 

 3), and the pore canal has again acquired an exterior opening (plate xxi, 

 figure 3; plate xx, figure 3). The parietal canal has opened into the oral 

 ccelom (plate xxi, figure 4) . In the aboral ccelom some irregularly arranged 

 trabecute are seen (plate xxi, figure 6). 



The primary gonad (Russo) is distinct (plate xxi, figures 3 and 6) ; the 

 axial organ has developed into a conspicuous string and the chambered organ 

 is very distinctly quinquelocular, with fairly large lumen (plate xx, figures 

 7 and 8; plate xxi, figures 3, 5, and 6). An interesting feature is seen in the 



