104 DR W. A. HERDMAN ON 



end of the branchial sac, which extends back usually to between the 5th and 

 6th muscle bands. Grobben, however, describes and figures* the peripharyngeal 

 band as lying in the 1st intermuscular space. Keferstein and Ehlers also 

 represent t the branchial sac as extending anteriorly into the 1st intermuscular 

 space, an arrangement which I have been unable to find in the "Triton" 

 specimens. 



The arrangement of the stigmata is as follows : — A series commences on 

 each side of the median dorsal line, close behind the 3rd muscle band (see 

 PL XVIII. figs. 8 and 11, sg), and extends posteriorly for a variable distance 

 — usually to about the 6th muscle band. The stigmata in this series differ 

 greatly in size among themselves. The most anterior one is very short — in 

 fact, almost circular. The next three or four increase rapidly in length till 

 the level of the nerve ganglion (n.g.) is reached, and then the increase becomes 

 less marked. Towards the posterior end there is a slight diminution in size. 

 Considered as a whole, the two series of stigmata diverge somewhat posteriorly, 

 so that the space between them in the dorsal middle line is narrow in the 3rd 

 intermuscular space, the region of the ganglion, but widens posteriorly (PI. 

 XVIII. fig. 11). As a result of this arrangement, when the branchial sac is 

 seen from the side, the dorsal series of stigmata appear to slope downwards and 

 backwards from the region of the ganglion (see PI. XVIII. fig. 4). There is also 

 a series of stigmata upon each side of the ventral median line. These, how- 

 ever, do not extend so far anteriorly as the dorsal series do. They commence 

 behind the 4th muscle band, near the posterior extremity of the endostyle, 

 and extend backwards, increasing in length rapidly at first, and then maintain 

 ing their size till they come to the sides of the oesophageal aperture. Here 

 they commence to curve dorsally, and then towards each other, finally uniting 

 in the dorsal middle line, usually near the 6th muscle band, so as to form a curve 

 surrounding the membranous area in which the oesophageal aperture is placed 

 (see PI. XIX. fig. 10, sg). 



The membranous side wall of the branchial sac is very wide anteriorly, 

 where it extends from the endostyle almost to the ganglion dorsally. In the 

 4th intermuscular space it is encroached upon by the development of the 

 ventral series of stigmata, and as it is traced posteriorly from this point, it 

 becomes narrower and narrower, till finally it merges upon each side into the 

 median dorsal area through the failure of the dorsal stigmata. The exact 

 number of stigmata in the different series varies of course according to the 

 size of the individual. In mature specimens there are usually from thirty 

 to fifty in the dorsal row on each side, and about thirty as an average in each 

 ventral series. 



* Loc. cit, p. 13, woodcut, and pi. i. fig. 1, wh. 

 f Zoologische Beitrtiga, pi. ix. figs. 1 and 2. 



