PTEROBRANCHIA. 
The minute individuals are associated 
together within a gelatinous investment ; 
the colony may attain a size of 9 in. 
by 6 in. The gut is curved, the anus 
being beside the mouth, beneath which 
are 4-6 pairs of arms with ciliated tent- 
acles. These two characters, formerly 
supposed to indicate Polyzoan affinities, 
may perhaps be adaptations to the sedent- 
ary life. With Balanoglossus this type has 
been compared, on account of the possession 
Fic. 239.—An individual Cephalo- 
discus.—After Ridewood. 
b., Buds: sé#., stolon; go., to the left, 
bulging of the body caused by the 
gonad ; ga., to the right, bulging of the 
body caused by the stomach ; J.s., pos- 
terior lobe of buccal shield; ~.2., a red 
line on the buccal shield; J.s., dark 
edge of the buccal shield ; 22. tentacular 
plumes. 
Fic, 238.—Piece of a colony 
of Cephalodiscus, showing 
the tubes inhabited by the 
animals. — After Ridewood. 
of the following characters :—(a) 
The body is divided into three 
regions, which correspond to the 
proboscis, collar, and trunk of 
Balanoglossus; this is especially 
obvious in the young bud; (4) 
each of the three regions contains 
a coelomic cavity, the most anterior 
being single, while the other two 
are divided by a median par- 
tition; (c) the anterior pre-oral 
cavity’ opens to the exterior by 
two pores (cf. proboscis pore 
of Balanoglossus); (d) the collar 
region is also furnished with two 
collar-pores; (2) in the collar 
region the dorsal nervous system 
is also placed, and is continued 
to some extent into the proboscis ; 
(f) beneath the nervous system 
lies a diverticulum from the gut, 
which extends towards the pro- 
boscis region; this has been 
compared to the ‘‘ notochord” 
of Balanoglossus ; (g) the anterior’ 
region of the gut is perforated by 
