54 



ORIGIN OF CHORDATES 



III. 2 



assists in the process of feeding, probably by serving to filter off the 

 excess water from the material already collected on the proboscis, 

 which often consists of large amounts of sand or mud. Relative to the 

 size of the animal the pharynx is less extensive than in amphioxus, 

 presumably because ciliary surfaces are provided on the outside and 

 also large masses of sand are forced into the mouth during locomotion. 

 There is no endostylar apparatus, but the ventral part of the pharynx 

 is often partly separated from the rest (Fig. 26). Along this groove the 

 matter ingested is passed to a straight oesophagus and intestine open- 

 ing by a terminal anus. There is no true tail in the adult but a post- 

 anal region is present in some species during development. Numerous 



v'.v. ph. 



Fig. 27. Diagram of the blood system of Balanoglossus. 



col. collar; d.v. dorsal vessel; glom. glomerulus; hp. hepatic caeca; m. mouth; not. 'notochord'; 

 p. proboscis; ph. pharynx; v.v. ventral vessel. (After Bronn.) 



hepatic caeca in the anterior part of the intestine can be seen from the 

 outside as folds of the body wall, often highly coloured. 



The blood system consists of a complex set of haemocoelic spaces, 

 communicating with large dorsal and ventral vessels (Fig. 27). 

 The former enlarges into a sinus anteriorly and this is partly sur- 

 rounded by the wall of a pericardial cavity, which contains muscles 

 and may be said to be the heart, though clearly lying in a very different 

 position from that of other chordates. From the sinus, vessels proceed 

 to the proboscis and round the pharynx to the ventral vessel. The 

 blood is said to move forwards in the dorsal and backwards in the 

 ventral vessels. The front of the sinus forms a series of glomeruli, 

 covered by a region of the proboscis coelom specialized to form 

 excretory cells, the nephrocytes, some of which drop off into the 

 coelom. The blood is red in some species but usually colourless. It 

 contains a few amoebocytes. 



The nervous system is one of the most interesting features of 

 Enteropneusta. It resembles that of echinoderms in consisting of a 

 sheet of nerve-fibres and cells lying beneath the epidermis all over the 

 body (Fig. 23). This sheet is thick in the mid-dorsal and mid-ventral 



