90 THE AMERICAN MUSEUM JOURNAL 



structure somewhat lower than that of a mollusk so far as complexity of 

 organization is concerned, they are actually primitive members of the 

 chordate group which includes the vertebrates and culminates in man. 

 Their true relationship is shown by their life history according to the 

 principle that an animal in its development tends to summarize certain 

 of the changes that its ancestors have undergone in evolution: their young 

 when first hatched are free-swimming, with tadpole-like tail, well-developed 

 median eye, "brain" and central nervous system of the primitive chordate 

 type, and the most striking indication of their affinity the larval notochord, 

 or cartilaginous prototype of a backbone. As the animal approaches 

 maturity however, it attaches itself to some stationary object, by means 

 of sucking disks upon its head. It then undergoes a metamorphosis which 

 involves the breaking up and degeneration of "brain," special sense organs 

 and notochord while the nervous system is reduced to little more than a 

 single ganglion. On the other hand the digestive system is rotated upon 

 itself and together with the circulatory system and other organs, becomes 

 adapted to the future sedentary life of the animal. 



As in the case of the hydroid, the organization of the adult ascidian is 

 chiefly that of a digestive and reproductive machine, but that it is of a 

 much higher type than the polyp is evident from the relatively high speciali- 

 zation of its digestive apparatus. 



Thus in an association of animals like that of the wharf piles, in which 

 the various species are adapted to a sedentary existence and a diet of 

 microorganisms, there may be three principal categories of forms, — first, 

 those like the polyps which have reached the adaptation as the result of an 

 evolution from still lower types; secondly, those like the tube-building 

 serpulid worms and the bivalve mollusks, which have become greatly 

 specialized to that mode of existence without much loss of complexity; and 

 thirdly, those like the ascidians, which have undergone a considerable 

 degeneration from a former more complex and highly organized type of 

 structure. 



Such an association also emphasizes the general tendency throughout 

 the great groups of animals to take advantage of all the available sources 

 of food supply in their environment. The microorganisms of the sea seem 

 especially suitable as prey for the smaller and more primitive members of 

 the animal kingdom like the sponges and polyps. Yet among the echino- 

 derms, worms, crustaceans, mollusks, ascidians, chordates and even fishes 

 (such as menhaden), there are certain forms in each group which have found 

 it of advantage to share this great abundance of food. Thousands of 

 microscopic creatures may swarm in a single drop of sea water; and in the 

 course of time the species that feed upon them have become more and 

 more perfectly adapted to this diet, often to the extent of undergoing pro- 

 found modifications of structure and habits of life. 



