ORIGIN OF CHORDATES 423 



pairs of pouches from the archenteron (strictly, two pairs and 

 an anterior median pouch which represents a fused pair). 

 Such coelomic sacs are enterocoelic, like the anterior gut- 

 diverticula of Amphioxus. The three sets of coelomic pouches 

 persist in the adult Balanoglossus, and it is interesting to notice 

 that the first two sets, forming the cavities of the " proboscis " 

 and of the " collar," have openings to the exterior. These 

 openings are coelomostomes, comparable to the water-pores of 

 the Echinoderms, and the connexions which are occasionally 

 found in Craniates between the premandibular somites and 

 the hypophysis (" proboscis-pores "). 



Allied to Balanoglossus are the Pterobranchia, which show 

 a slight trace of a notochord, but no dorsal tubular nerve- 

 cord. One of them, Cephalodiscus, has the three sets of 

 coelomic pouches, each with a coelomostome, and a pair of 

 gill-slits. It is not free-swimming but sessile, reproducing 

 actively by budding. The other, Rhabdopleura is not only 

 sessile but colonial, for the buds formed remain in con- 

 nexion with the parent stock. Rhabdopleura has the three 

 sets of coelomic pouches, and coelomostomes, but no gill- 

 slits. 



The next form to consider is Phoronis, which is worm- like 

 with the anterior end modified into a row of tentacles. The 

 anterior region of the body corresponding to the proboscis is 

 reduced to a flap overhanging the mouth, so that the body 

 contains only two sets of coelomic pouches. The larval form 

 of Phoronis which is called the Actinotrocha, has ciliated bands 

 reminiscent of those of the Tornaria. Phoronis has nephridia, 

 and as these structures are also present in Amphioxus, it is 

 possible that they were present in the original common ancestor 

 from which all these forms were descended. 



Phoronis is related to the Ectoproctous Polyzoa, and to 

 the Brachiopoda. All these forms, so far as is known, tend to 

 have coelomic pouches developed as enterocoels, and usually 

 showing a tripartite arrangement. Many of them have open 

 coelomostomes. The larval forms usually have a ciliated band 

 passing behind the mouth, and cleavage of the egg is in- 

 determinate. These features distinguish the chordates and 



