TYPE PROTOCHORDATA. 



597 



terior portion of the digestive tract arid containing a more or 

 less perfect cavity which communicates with the oesophagus. 

 The body is divisible into three regions : an anterior prseoral 

 epistonie or proboscis, a median collar region, and a posterior 

 visceral sac or trunk region. The coelom is divided into 

 corresponding regions, a pair of pores placing the collar com- 

 partment in communication with the exterior, while one or 

 two additional pores perform a similar office for the epistome 

 or proboscis cavity. The nervous system remains in connec- 

 tion with the ectoderm, and its principal portion is situated 

 in the collar region. 



1. Order Pterobranchia. 



This order contains but two genera, Rhabdopleura and 

 Cephalodiscus, the former occurring in deep water off the coast 

 of Norway, while the latter was obtained by the " Challenger " 

 Expedition in the Straits of Magellan. 



Rhabdopleura (Fig. 271) is a colonial form, consisting of a 



FIG. 271. COLONY OF Rhabdopleura (after LANKKSTER). 



stolonlike system of tubes ramifying over the surface of 

 stones, etc., and giving off shorter lateral tubes each one of 

 which contains an individual. The tubes are composed of 

 chitiulike material and form a "house" for the colony, and 



