TYPE PBOTOCEOBDATA. 603 



of several different organs. The centre of tbe mass is occu- 

 pied by the notochord (nc), which is, as in the Pterobranchia, 

 a forwardly-directed diverticulum of the dorsal wall of the 

 anterior portion of the digestive tract and contains a more or 

 less distinct cavity communicating with that of the oesophagus. 

 Dorsal to the notochord lies a contractile heart (h), and 

 dorsal to this again, and surrounding it, a sac (ps) containing 

 a few, or in some cases many, cellular elements. This pro- 

 boscis-vesicle, as it has been termed, may possibly represent a 

 portion of the proboscis- coelom, which would then consist of 

 two separate cavities, originally right and left, one of which 

 becomes very large and fills the greater portion of the pro- 

 boscis, while the other remains quite small. Surrounding 

 these structures are a number of folds of the splanchnic 

 layers of the proboscis-coelom {"pg), loops of blood-vessels 

 lying in the folds, while the cells covering them frequently 

 contain yellow granules. It has been supposed that these 

 granules indicate a glandular function for the cells, and con- 

 sequently the entire mass of folds has been termed the pro- 

 boscis-gland. 



The ccBlomic cavities of the collar (Fig. 275, B cop) and 

 trunk are much simpler and are paired, the cavities of the 

 right and left sides being separated from each other by dorsal 

 and ventral mesenteries. From the dorsal portion of the an- 

 terior end of the trunk-coelom two prolongations extend for- 

 ward into the collar, lying on each side of the dorsal blood- 

 vessel and forming the perihsemal cavities in the interior of 

 which are longitudinal muscle-fibres. Two other similar for- 

 ward prolongation of the trunk-coelom lie between the collar 

 coelom and the oesophagus, forming the peripharyngeal cavi- 

 ties. 



In contradistinction to what occurs in the Pterobranchia, 

 a well-developed blood system is present, consisting in the 

 collar and trunk of a dorsal and ventral longitudinal vessel 

 with distinct muscular walls and lying in the mesenteries. 

 From the dorsal vessel branches pass to the tonguelike pro- 

 cess of the branchial slits, and the vessel itself is continued 

 forward into the proboscis to enter a space between the pro- 

 boscis-vesicle and the notochord which is termed the heart. 



