258 MORPHOLOGY OF INVERTEBRATE TYPES 



the wall of the tunique. They communicate directly with the 

 schizoccele. The hypodennis (called also epidermis), which 

 secretes the tunique, is formed by a single layer of cylindrical 

 epithelial cells. Under the hypodermis are longitudinal and 

 circular muscle fibres especially well developed in the siphons. 

 The two siphons are formed by an infolding or invagination of 

 the body wall and are lined, therefore, with a cuticle which is an 

 equivalent of the tunic. Inside the incurrent siphon, at its base, 

 is a crown of twelve branched tentacles. Four of these are of 

 the first order and are so situated that two are in the plane of 

 symmetry and two at right angles to it. The remaining eight 

 tentacles are of the second order and much smaller than the 

 former. These tentacles form a transverse sieve at the bottom 

 of the siphon and undoubtedly help to keep larger foreign bodies 

 from entering the pharynx. The excurrent siphon has no ten- 

 tacles. 



The plan of structure of Molgula is a strange mixture of per- 

 fect symmetry with asymmetry. The former is the original 

 plan, apparent from the structure of the pharynx, the relative 

 position of the nervous system and endostyle to the siphons 

 and of the gonads to the endostyle. But the immense develop- 

 ment of the pharynx and its peculiar function as an organ of 

 respiration has brought about the displacement of the rest of 

 the alimentary canal to the left side of the body and of the heart 

 and kidney to the right side. The plane of symmetry bisects 

 the endostyle and both siphons. The endostyle is ventral, while 

 the nervous system, situated between the siphons, is dorsal in 

 position. 



In order to understand the structure of Moigula, one must 

 remember that the pharynx has developed enormously at the 

 expense of all other organs. Disregarding for a moment the 

 siphon and cloaca, one might compare the animal to a rubber 

 ball in which the inner space represents the pharyngeal cavity, 

 while all the organs are in the wall. The difference lies in the 

 fact that in Molgula this wall is not solid. The organs lie, in 



