168 ARCHICGELOMATA., 
Many bryozoan colonies have a close superficial resemblance to the 
hydroid colonies, hence it should be noted that the bryozoan polype is 
a far more highly organised animal than the hydroid. The possession 
by the former of mesoderm and a ccelom and a definite nervous system 
may be specially emphasised. 
IV.—SAGITTA. 
PHYLUM ARCHICGLOMATA (p. 170). 
SuB-PHYLUM CHA&TOGNATHA (p. 177). 
Sagitta, the arrow-worm, is a free-swimming pelagic animal of 
elongated body and may be about 3{ inch in length. It is one of the 
simplest and best types of the pelagic zekton. Its body is of a glassy 
transparency, cylindrical in transverse section and perfectly plano- 
symmetric. The anterior end is formed into a head, with mouth 
surrounded by tufts of sete or bristles which act as jaws. The 
posterior end bears a bifid caudal or tail-fin and two pairs of lateral 
fins protrude from the body. 
Three parts of the body can be distinguished. The head, the 
elongated zvunk and the faz/. The mouth leads into a pharynx, which 
passes into a simple intestine, terminating in an anus, situated ventrally 
between trunk and tail. Corresponding with the three segments are 
the three mesodermic segments. The head segments have their walls 
largely modified into jaw-muscles; the trunk segments also form 
dorsal and ventral longitudinal muscles and a pair of spacious coelomic 
cavities. The walls of these form dorsal and ventral mesenteries 
supporting the intestine. In the tail the segments are very similar, 
but as there is no intestine in this part the mesentery is continuous and 
median. 
In the trunk the ccelomic walls form paired lateral ovaries; in the 
tail they form ¢estes. Each of these lead, by paired oviducts and vasa 
deferentia respectively, to the exterior near the anus. The animal is 
therefore hermaphrodite. Transverse septa are found between the 
three segments. 
The nervous system consists of a dorsal brain in the head with paired 
connectives round the neck to a large subcesophageal mass on the 
ventral surface of the trunk. The brain supplies nerves to a pair of 
simple eyes on the head and certain sense-papille. 
Sagitta reproduces only sexually. The eggs and larve are pelagic 
and transparent, though demersal eggs and larvee are known in the 
sub-phylum. 
V.—WALDHEIMIA. 
PHYLUM ARCHICCELOMATA (p. 170). 
Sus-PHYLUM BRACHIOPODA (p. 177). 
Waldheimia is a small marine organism enveloped in two shells. 
They are roughly circular in outline and convex externally. The so- 
called ventral shell is produced behind the other or dorsal into a 
bee be 
