xiii PHYLUM CHORDATA 5 



superficial and does not correspond to an internal segmentation, 

 being most strongly marked behind. The coelome of the trunk 

 is divided into two lateral closed cavities by a vertical partition 

 (dorsal and ventral mesenteries). 



Digestive Organs. The mouth (Fig. 706, mo.) is situated 

 ventrally at the base of the proboscis, within the collar. Into the 

 dorsal half of the anterior portion of the alimentary canal open 

 the internal gill-openings. Each of these is in the form of a long 

 narrow U, the two limbs separated by a narrow process the tongue 

 which contains a prolongation of the body-cavity. In most of 

 the Enteropneusta the internal gill-openings lead into gill-pouches 

 which in turn communicate with the exterior by the gill-slits. 



v 



e*' 



FIG. 707. Ptychodera bahamensis. Transverse section of the branchial region. 6. 

 branchial part of alimentary canal ; b. , c 3 , coelome of trunk ; d. m. dorsal mesentery ; d. n. 

 dorsal nerve ; d. v. dorsal vessel ; e. epidemis with nerve layer (black) at its base ; g. genital 

 wing ; g. p. branchial aperture encroached upon by tongue (t) ; 1. lateral septum ; m. longi- 

 tudinal muscles ; o. digestive part of oasophagus ; r. reproductive organ ; t. tongue ; v. ventral 

 mesentery and ventral vessel ; v. n. ventral nerve. (From Harmer, Cambridge Natural History, 

 after Spengel.) 



But in the genus Ptychodera (Fig. 707) there are no gill-pouches, 

 the U-shaped internal gill-openings leading directly to the exterior. 

 The gill-pouches are supported by a chitinoid skeleton consisting 

 of a number of separate parts. Each of these consists of a- dorsal 

 basal portion and three long narrow lamella, a median and two 

 lateral ; the median, which is bifurcated at the end, lies in the 

 se'ptum or interval between two adjoining gill-sacs ; the two 

 lateral lie in the neighbouring tongues. In most species a number 

 of transverse rods the synapticulce connect together the tongues 

 and the adjoining septa, and are supported by slender processes of 

 the skeleton. 



The posterior part of the alimentary canal is a nearly straight 

 tube, giving off in its middle part, paired hepatic cceca, which bulge 



