XIII 



PHYLUM CHORDATA 



5 



are two rows of prominences (hep.) formed by the hepatic caeca. 

 The trunk is irregularly ringed, this annulation, which is entirely 

 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. 715, 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 (J , the two limbs separated by a narrow process the tongue 

 which contains a 

 prolongation of the 

 body-cavity. In 

 most of the Entero- 

 pneusta the internal 

 gill - openings lead 

 into gill-pouches 

 which in turn com- 

 municate with the 

 exterior by the gill- 

 slits. But in the 

 genus Ptychodera 

 (Fig. 716) there are 

 no gill-pouches, the 

 U -shaped internal 

 gill-openings leading 

 directly to the ex- 

 terior. The gill- FIG. 716. Ptychodera bahamensis. Transverse section of 



pouches are sup- 

 ported 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 synapticulcs 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 

 outwards in the series of external prominences already mentioned. 

 Posteriorly it terminates in an anal aperture situated at the 

 posterior extremity of the body. In the posterior part of its 



.c'- 



TTV 



the branchial region, b. branchial part of alimentary canal ; 

 b. c :i , coelome of trunk ; d. m. dorsal mesentery ; d. n. 

 dorsal nerve ; d. a. dorsal vessel ; e. epidermis with nerve 

 layer (black) at its base ; g. genital wing ; g. p. branchial 

 aperture encroached upon by tongue (t) ; I lateral septum ; 

 m. longitudinal muscles ; o. digestive part of oesophagus ; 

 r. reproductive organ ; t. tongue ; v. ventral mesentery 

 and ventral vessel ; v. n. ventral nerve. (From Harmer, 

 Cambridge Natural History, after Spengel.) 



