106 Biology in America 



to the ancestral form, but not in the direct line of evolution 

 of any of the higher types of animals. 



Progressing upward from our ancestral trochophore, we 

 come to forms whose general plan of structure is that of a 

 double cylinder, somewhat flattened along one axis, with two 

 sides alike giving it bilateral symmetry, and the upper or 

 dorsal differentiated from the lower or ventral surface. The 

 anterior end of this cylinder is differentiated as a head, with 

 mouth, sense organs and primitive brain, while the posterior 

 end lacks sense organs and contains an anus. The outer wall 

 of this double cylinder is made up of "skin" and muscles, 

 the inner is the wall of the digestive tract, with a body 

 cavity or coelome between. The cylinder is further divided 

 into rings or segments, each of which bears one or more pairs 

 of appendages for locomotion. 



Of such a form the annelid worm, of which the common 

 angle worm is an example, is typical, and from here onward 

 in our progress to the vertebrates and man we are on surer, 

 though still insecure footing. The annelid or annulate is as 

 its name indicates a series of joints or rings, all more or 

 less alike, which are very numerous in the typical form, but 

 in the more specialized groups are greatly reduced. In fact, 

 evolution from the annelid upward consists very largely in 

 the concentration and specialization of these segments. While 

 the annelids are probably not to be regarded as the direct 

 ancestors of the vertebrates, their general plan of structure, 

 with a coelome and a segmented body, is fundamental to that 

 of all higher types, even though it may be greatly modified 

 in some. 



The origin of the vertebrates is still shrouded in the mist 

 of hypothesis and dispute, and indeed may ever remain so, 

 since their invertebrate ancestors probably no longer exist. 

 Nevertheless a clue to their origin may be found in the 

 tunicates described above and in a little animal with a big 

 name, Balanoglossus, or the acorn-tongue. This is a worm- 

 like creature living in burrows in the mud or sand of the sea 

 bottom near low tide level, and in its development passing 

 through a trochophore stage. The anterior end is marked by 

 a contractile and very sensitive proboscis, from the shape of 

 which the animal derives its name, behind which is a collar 

 region, which in turn is followed by the trunk. Between the 

 proboscis and collar opens the ventral mouth which leads into 

 the pharynx, which is perforated by a large number of paired 

 openings to the exterior, the gill slits. Dorsal to the mouth 

 is a diverticulum of the pharynx which projects into the pro- 

 boscis, and which Bateson has identified as the notochord. 

 The nervous system presents features of peculiar interest. 



