14 
- 
HEMICHORDATA 
CHAP. 
of the dorsal body-wall, a conspicuous external feature of the 
species in which they are present. 
7.—Schizocardium brasiliense ; longi- 
tudinal, median section through the pro- 
boscis, the collar, and the first part of the 
trunk; 6, main blood-space of the pro- 
boscis ; b.c!, b.c?, b.c®, first, second and 
third body-cavities ; ¢.m, circular muscles 
of proboscis ; e, epidermis; /.m, longi- 
tudinal muscles of proboscis ; m, mouth ; 
n, notochord ; .s, central nervous system, 
continuous with the subepidermic nerve- 
plexus (black) of the proboscis, and with 
the dorsal nerve (d@); p.c, pericardium ; 
p-s, proboscis-stalk ; s, proboscis -skele- 
ton; v, vermiform process of notochord. 
(After Spengel.) 
The most interesting 
peculiarity of the digestive 
tract in this region is the 
existence, in certain species, 
of pores, possibly vestigial gill- 
slits, leading from it to the 
exterior. 
Notochord and Skeleton. 
—The structure compared by 
Bateson with the Vertebrate 
notochord is a hollow dorsal 
outgrowth of the alimentary 
canal of the collar-region (Fig. 
7, n). Near its origin it is 
slender, but in the proboscis 
it dilates into a comparatively 
large organ, which in most 
cases retains its cavity. Its 
cells have a vacuolated appear- 
ance, which recalls the fine 
structure of the Vertebrate 
notochord. In Schizocardiwm 
and Glandiceps, the organ is 
produced into a slender “ ver- 
miform process” (v), which 
extends nearly to the tip of 
the proboscis. 
The main support of the 
proboscis-stalk is the “ pro- 
boscis - skeleton” (s), a A- 
shaped organ whose median 
part lies beneath the base of 
the notochord, its diverging 
legs extending backwards 
along the outer side of the 
alimentary canal of the collar. 
The proboscis-skeleton, lke 
the branchial skeleton, is a special development of the structure- 
less membrane which is found at the base of the layers of cells 
