198 MORPHOLOGY OF INVERTEBRATE TYPES 



bulacral tubes are muscular structures covered externally as 

 well as internally with ciliated epithelial cells. 



Respiratory system. The five pairs of branchiae already 

 mentioned function as organs of respiration as do possibly also 

 the ambulacral tentacles of the aboral surface. The branchiae 

 are outgrowths of the peristomial membrane and communicate 

 with the oral sinus. 



Circulatory system. The circulatory organs are composed 

 of a system of sinuses and a system of lacuna. The system of 

 sinuses is characterized by a complete isolation of its component 

 parts. The five radial sinuses end blindly at both ends. The 

 oral sinus has been described above and communicates only with 

 the five pairs of branchiae. The aboral sinus communicates only 

 with its five prolongations, the genital sinuses. The axial sinus 

 is reduced to the central cavity of the axial organ and commu- 

 nicates with the hydrophoric canal. The system of lacunae re- 

 sembles that of the starfish, though somewhat complicated by 

 the presence of stomachal lacunae. The axial organ which is part 

 of the system of lacunae, communicates with the aboral as well 

 as the oral circular lacuna. The latter surrounds the oesophagus. 

 Five radial lacuna run from the oral lacuna to the terminal ten- 

 tacles. The highly branched system of stomachal lacuna ex- 

 tends over the entire stomach and part of the intestine. It con- 

 sists of two canals with small branches opening into them along 

 their entire extent. One of these canals is the external stomachal 

 lacuna, the other the internal stomachal lacuna. They are closely 

 applied to the stomach and the internal lacuna communicates 

 with the oral lacuna. The aboral lacuna gives rise to five genital 

 lacunae. 



Nervous system. The nervous system includes, as in the 

 starfish, three distinct systems. The ectoncural system consists 

 of an oral ring surrounding the mouth and situated on the inner 

 surface of the lip, five radial nerves with their branches and a 

 subepidermal nervous plexus. The radial nerves are not super- 

 ficial but run along the inner surface of the test in so-called 



