Be.“ 
External Characters. 
Figures 5 and 6 representing respectively tbe entire body and 
the head portion of Geotria australis in lateral aspect are self-explanatory 
(cf. fig. 17 of Prare 49). This figure has been drawn to scale with 
great care and, among other features, represents accurately in one of 
my specimens the irregular position (somewhat variable in different 
specimens) of all the lateral line sense organs on the body, except 
perhaps on the gular sac, in which region the sense organs were not 
easy to distinguish in my spirit material. The most conspicuous parts 
of the animal are of course the huge suctorial funnel and the disten- 
sible gular pouch or sac. This gular sac (figs. 6, 8) has been known 
for many years to contain a spacious blood sinus,t) since MILNE 
Epwarps (47) injected it from the vascular system before 1858, and 
Huxuey (21) described it as containing blood in the specimens he 
examined. I have also found plenteous blood contents in the sac. 
The wall of the gular sac is fairly thick and muscular (especially in 
the middle line of the front wall), the blood space of course lying 
between this and the ventral muscles of the head (cf. the ventral lymph 
sacs of the frog), Anteriorly the blood space is traversed by some 
strands of connective tissue, also by a few fine nerves supplying the 
wall muscles. The precise function of this peculiar gular sac is, so 
far as I am aware, not known. It appears to be certain that when 
in use it becomes greatly distended but it is not clear in what way 
this assists the process of feeding or attachment or any other habit of 
the animal. Figure 7 shows an enlarged view of one of the gill- 
clefts and shows the small upper and lower processes which, if they 
joined, would divide the cleft into anterior and posterior parts; these 
processes are however quite freely movable and can hardly be regarded 
either as related to the inhalent and exhalent functions of the cleft 
or as a valvular apparatus. A depression of the skin is situated im- 
mediately behind the actual cleft and enclosed in the same oval de- 
pression of the external skin. 
Fig. 8 represents the ventral aspect of the head, including the 
appearance and arrangement of the ‘“ maxillary” and other teeth of 
the disc. The cavity of the mouth is left unshaded. Round the 
1) Not a lymph space, as supposed by PLATE, or an outpushing of the 
gut, as supposed by others. The cavity of the gular sac is, as SMITT points 
out, represented in Petromyzon marinus by a narrow space lying under the 
skin in the throat region. 
