xm PHYLUM CHORDATA 3 



tudinal ridges the genital ridges (gen.) not recognisable in some 

 species, extend throughout a considerable part of the length of the 

 body both behind and in the region of the gill-slits (branchial 

 region); these are formed by the internally situated gonads. 

 Behind the branchial region are two rows of prominences (hep.) 

 formed by the hepatic cceca. The trunk is irregularly ringed, this 

 ringing, which is entirely superficial and does not correspond to 

 an internal segmentation, being most strongly marked behind. 

 The ccelome of the trunk is divided into two lateral closed cavities 

 by a vertical partition (dorsal and ventral mesenteries). 



Digestive Organs. The mouth (Fig. 665, 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. 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 lamellae, a median and two 

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

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

 lateral lie in the two neighbouring tongues. In some species a 

 number of slender transverse rods the synapticulce connect 

 together the tongues and the adjoining septa. 



The posterior part of the alimentary canal is a nearly straight 

 tube with, 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. Throughout its length it lies 

 between the dorsal and ventral divisions of the vertical partition, 

 which act as mesenteries. 



Skeleton. In front the dorsal wall of the anterior portion of 

 the alimentary canal gives off a diverticulum (div.), the lumen of 

 which extends nearly to the anterior end. This diverticulum 

 consists of epithelium with gland cells and of a sort of retiform 

 connective tissue ; it has been supposed to be homologous with 

 the notochord of the typical Chordata. In close relation with this 

 on its ventral surface is the proboscis-skeleton (prob. sJcel.) which 

 consists of a median part, of an hour-glass shape, with a tooth- 

 shaped process, bifurcating behind into two flattened bars which 

 lie in the anterior region of the collar and support the opening 

 into the lumen of the diverticulum. 



There is a blood- vascular system with dorsal and ventral 

 longitudinal trunks. The dorsal vessel (dors.v.) lies above the 

 notochord, and ends in front in a sinus situated in the anterior 

 part of the collar and the neck of the proboscis. From the pos- 

 terior part of the sinus is given off a vessel which bifurcates to 



B 2 



