242 bulletin: museum of comparative zoology. 



lenger," it was very small as compared with the size of the animal as a 

 whole. It was iu the form of a nearly spherical mass, and was situated 

 underneath the posterior part of the oral disc. I consider it quite likely 

 that in life the mass is considerably more elongated, and extends through 

 a greater portion of the distance between the two orifices. But this is 

 entirely conjecture, so the question need not detain us now. 



The most noteworthy thing pertaining to the intimate structure of 

 the visceral mass is the fact that the true hranchial sac is situated within 

 it, or constitutes a part of it. An understanding of the orientation and 

 structure of the mass becomes clear by examining the figures in the 

 order mentioned, 8, Plate 3, 3, 4, and 5 Plate 2. The dissection from 

 which Figure 8 was drawn is seen at once by comparing this figure with 

 Figure 1. From these it will be seen that the branchial orifice opens 

 directly into a capacious tube, in reality the homologue of the inner part 

 of the branchial siphon of ordinary ascidians. This is shown as cut in 

 Figure 8. On the ventral side it carries the longitudinal muscle bands 

 m. b". of Figure 1, and shown from within in Figures 3 and 4. Tlie 

 ventral wall of this tube, it will be seen, passes directly over into the so- 

 called horizontal membrane (h. m., Fig. 3). On the dorsal surface of the 

 mass (Fig. 8) one sees the rather conspicuous ganglion {n. g.), the sub- 

 neural gland (s. gl.), the dorsal half of the peripharyngeal band (p. b. b.), 

 and on close examination, tlie dorsal lamina (d. I.). All these organs 

 are situated on what seems, from a dissection like that shown in Figure 

 8, to be the relatively very large stomach. At the posterior margin of 

 the mass are seen the ovary (o?;.) and testis (tes.). 



Figure 3, Plate 2, represents the visceral mass of the same specimen 

 as that shown in Figure 8, but removed from the test, enlarged consid- 

 erably, and with the piece carrying the ganglion, gland, etc., cut out. 

 Examination of the piece on its inner surfoce, with a low power, reveals 

 the fiict that two distinct membranes enter into its composition, and 

 that the inner of these is perforated by a considerable number of ellip- 

 tical but more or less irregular orifices (Fig. 9, Plate 3). Perforations 

 of the same sort were also found later on the portion of the wall not 

 cut away (Fig. 3, br. s.). Each orifice is bordered by a rather decided 

 epithelial thickening, the inner margin of which has few, but the outer 

 very many, nuclei (Fig. 6, Plate 2). I have found no intimation of 

 ciha fringing the orifices. The absence of cilia on the apertures, the 

 somewhat peculiar structure of their bordering epithelium, and their 

 irregular distribution, are traits in which they differ considerably from 

 the more typical branchial stigmata of ascidians. Nevertheless there 



