XIII 



PHYLUM CHORDATA 



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, 7no.) 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 those is in the form of a long 

 narrow U, the two limbs separated bva 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. 



Fig. 707. — Ptychodera bahaxnensis. Transverse section of the biancliial region. 6. 

 branchial I'art uf alimentary canal ; 6., <•', ccelome of trunk; (/. iit.. dorsal mesentery ; d. n. 

 dorsal nerve ; d. r. dorsal vessel ; e. epidemis with nerve layer (black) at its base ; <j. genital 

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

 tudinal muscles ; o. digestive part of (esophagus ; /•. 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 PtycJiodcra (Fig. 707) there are no gill-pouches, 

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

 The gill-pouches are sujsported by a chitinoid skeleton consisting 

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

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

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

 septum or interval between two adjoining gill-sacs ; the two 

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

 of transverse rods — :the synaptkuke — 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 ncarlj'^ straight 

 tube, giving off in its middle part, paired Jicpaiic cwca, which bulge 



