Notes on the morphology of the Tunicata. ' 5*75 



separated by a sliort interval from its anterior end, lies the ciliated 

 funnel (see Section I, page 549 et seq.). 



The presence of endostyle, dorsal lamina and ciliated funnel in 

 this part of the digestive tube is enough to show that it is not merely 

 the oesophagus, but is really a modified posterior portion of the 

 pharynx. There is one other feature of even greater interest, which 

 points in the same direction. On the ventral surface of the visceral 

 mass, as seen in exterior view, are two oval perforations (Figs. 81 

 B and C and 85 g.s). In my former brief paper I described these 

 as perforations of the perivisceral membrane. More careful examination 

 shows that they extend not only through this membrane but through 

 the wall of the digestive tube as well. I have found them in three 

 of the least injured of my specimens, in exactly the same position. 

 I cannot say from study of sections whether the edges of these per- 

 forations bear cilia or not. These apertures, I believe, function as 

 gill slits. Whether they are modified stigmata, or new structures 

 having a similar function, one cannot say. 



Of the mesodermal organs, the muscles and the gonads are of 

 chief interest. The muscles of the horizontal membrane are similar 

 to those of 0. Uthyus, except that at the tips of the lobes the trans- 

 verse strands lose their definite arrangement like the rungs of a 

 ladder and form, instead, a lattice-like meshwork. The peculiar 

 muscles on the (morphologically) dorsal surface of the visceral mass 

 have an arrangement as shown in Figs. 81 B and 84. These are 

 probably homologous to portions of the intersiphonal muscular system 

 of the Ascidians, though in Octacnemus they are modified, in part 

 probably, to aid in keeping up the currents of water through the 

 body. I have just referred to the gill slits. It is easy to see that 

 these muscles, lying close over the dorsal surface of the digestive 

 tube, would, by their contraction, contract the cavity of this tube, 

 and thus aid the passage of water through the gill slits. What 

 further function, if any, they may have, it is difficult to say. The 

 two delicate longitudinal muscle bands on the median line (shown 

 in Fig. 84 r.m) are the two rapheal muscles. In the Ascidians, the 

 rapheal muscle is double, at least at its upper end. In Octacnemus 

 it is double throughout. The muscles of the mantle are very weakly 

 developed. 



The gonads are very difficult to study, for my specimens are 

 too soft for successful dissection. I find a single large ovary and 

 a still larger testis, but am unable to make out their ducts. (Cf. 



Zool. Jahrb. Xni. Abth. f. Morph. ßg 



