in. 10 LARVAL ANCESTRY OF CHORDATES 75 



swimming, fish-like animal. The chordates are related to the echino- 

 derms and their allies. This is established by the similarities in early 

 development (cleavage, gastrulation, mesoderm formation); by the 

 presence in early members of both groups of three separate coelomic 

 cavities, some with pores; by the similarity of the larva of entero- 

 pneusts to the dipleurula, and by other points of general morphological 

 and biochemical similarity between early chordates and echinoderms, 

 especially the arrangement of the nervous system and presence of a 

 mesodermal skeleton. 



The echinoderms we have to consider are not the modern star- 

 fishes and sea-urchins, which are relatively active animals, but their 

 sessile Palaeozoic ancestors. These were sedentary, often stalked 

 animals, the cystoids, blastoids, and crinoids, feeding by ciliary 

 action. Surviving animals of related phyla, such as Polyzoa Ecto- 

 procta and Phoronis suggest that the ancestor for which we are looking 

 may have possessed a ciliated lophophore for food-collecting. For 

 purposes of dispersal its life-history presumably included a larval 

 stage with a longitudinal ciliated band, similar in plan to that of the 

 auricularia. 



One might well ask how such an animal could possibly become 

 converted into a motile, metameric fish, feeding with its pharynx. 

 Yet the evidence of the lower chordates is sufficient to establish that 

 this change has occurred, and even provides us with an outline of the 

 main stages in the process of the change. Cephalodiscas, which is in 

 some ways the most primitive of surviving chordates, with its lopho- 

 phore also possesses gill-slits. This suggests that the pharyngeal 

 mechanism was substituted for the lophophore as a means of feeding 

 in the adult stage. There are other possible interpretations. It has 

 been suggested that Cephalodiscus was derived from a larval entero- 

 pneust (Burden-Jones). However, it is possible that ciliary mechanisms 

 developed in the pharynx first to deal with food collected outside by 

 tentacles or proboscis. Later the pharynx became developed into a 

 self-contained feeding mechanism, making unnecessary the tentacles, 

 which provide a tempting morsel for predators. The adoral band of 

 cilia of the auricularia probably serves to carry food into the mouth, 

 and for this purpose it is actually turned in to the floor of the pharynx. 

 Garstang suggests that the endostyle has been derived from this loop 

 of the adoral band. 



The pharyngeal method of food-collecting thus replaced the ten- 

 tacles in the adult and the whole apparatus of an endostyle and an 

 atrium to protect the gills became developed. We may notice here the 



