i68 ARCHICCELOMATA. 



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 coelom and a definite nervous system 

 may be specially emphasised. 



IV.— SAGITTA. 



Phylum - - - Archiccelomata (p. 170). 

 Sub-Phylum - - Ch/Etognatha (p. 177). 



Sagitta, the arrow-worm, is a free-swimming pelagic animal of 

 elongated body and may be about ^ inch in length. It is one of the 

 simplest and best types of the pelagic nekton. Its body is of a glassy 

 transparency, cylindrical in transverse section and perfectly piano- 

 symmetric. The anterior end is formed into a head, with mouth 

 surrounded by tufts of setse 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 trunk and the tail. The mouth leads into a pharynx, which 

 passes into a simple intestine, terminating in an amis, 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 coelomic walls form paired lateral ovaries ; in the 

 tail they form testes. 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 doj^sal brain in the head with paired 

 connectives round the neck to a large suboesophageal mass on the 

 ventral surface of the trunk. The brain supplies nerves to a pair of 

 simple eyes on the head and certain sense-papillae. 



Sagitta reproduces only sexually. The eggs and larvae are pelagic 

 and transparent, though demersal eggs and larvae are known in the 

 sub-phylum. 



v.— WALDHEIMIA. 



Phylum - - - Archiccelomata (p. 170). 

 Sub-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 



