436 SUB-PHYLUM HEMICHORDA. 
beneath the epidermis. There are no special sense organs 
in the adult. In the larvae of some species there are two 
eye-spots. 
Alimentary system.—The permanently open mouth is 
on the ventral surface between the proboscis and the collar. 
Sand seems to pass into it during the wriggling movements 
of the animal, which are greatly aided by the turgidity of 
the proboscis and collar. The pharynx is divided into a 
dorsal and ventral region, of which the former is respiratory 
(Fig. 235, g.1), and connected with the exterior by many 
gill-slits, while the latter is nutritive (Fig. 235, g.), and 
conveys the food particles onwards. Behind the region 
with gill-slits, the gut has a dorsal and a ventral ciliated 
groove, and bears, throughout the anterior part of its course, 
numerous glandular sacculations, which can be detected 
through the skin. The anus is terminal. The animal eats 
its way through the sand, and derives its food from the 
nutritive particles and small organisms therein contained. 
Skeletal system.—The skeletal system is represented by 
the “notochord,” which lies in the proboscis, and arises, 
like the notochord of indubitable Vertebrates, as a diverti- 
culum from the dorsal wall of the gut in the collar region. 
Beneath the notochord there is a chitinous ‘proboscis 
skeleton.” The septa between the gill-slits are supported 
by chitinous “forked primary” bars; and each slit, at first 
circular, is split into a V-shape by the growth downwards 
of a double rod of chitin called a “tongue bar”; the whole 
is suggestive of Amphioxus. 
The body cavity.—The body cavity consists of five 
distinct parts, all of which are lined by mesoderm, and 
arise as pouches from the archenteron. (a) There is first 
the unpaired cavity of the proboscis, which communicates 
with the exterior by a dorsal pore at the' base of the pro- 
boscis next the collar. (d) In the collar region there are 
two small paired coelomic cavities, from which two funnels 
open to the exterior. Both these cavities and that of the 
proboscis tend to be obliterated by growth of connective 
tissue. (c) Two other cavities extend along the posterior 
region of the body, to some extent separated by the dorsal 
and ventral mesentery which moors the intestine. In these 
there is a body cavity fluid with cells. 
