Phylogeny of Animals 541 



growth of the notochord in the cephahc region, has become 

 abstricted off between the outer and inner muscular layers. 

 The paired "second body-cavities" or "collar cavities" would 

 similarly represent lateral coelomic enlargements behind the 

 region of the proboscis attachment, that resulted from passage 

 outward permanently of the entire proboscis and enlargement 

 for absorption of water, as well as muscular strengthening 

 of the collar area, as a mechanical basis for the proboscis. 

 The paired posterior body cavities would again represent 

 abstrictions round the area behind which originally the re- 

 tracted proboscis lay. The accompanying diagrams illustrate 

 such a graded transition series as would be involved in the 

 above description (Fig. 26 a-d). 



The hemichordal brain indicates alike derivation from an 

 advanced nemertean ancestry, and degradation of the main 

 nerve centers. For it is largely located on the dorsal surface, 

 and there also gives off a strong median dorsal branch. But 

 its diffuse flattened aspect, its loss of definite brain lobes, and 

 its continued communication by neuropores with the epidermis, 

 from infolding of which it is derived, all indicate simplifying 

 and diffuse departure from a primitively higher type of or- 

 ganization. 



The gradual expansion and diffusion of the dorsal and ven- 

 tral ganglionic centers of nemertean type over the enlarging 

 collar, and the shifting backward, during the process, of the 

 ganglionic ring, till it occupies a position in the posterior part 

 of the collar, in Balanoglossus, are evidently correlated with 

 the increased use of the permanently exserted proboscis as a 

 burrowing organ, and the utilization of the collar as a mechani- 

 cal, muscular, sensory, and respiratory basis for the proboscis. 

 The gradual shortening of the invaginated part of the probos- 

 cis, the conversion of the muscular and nervous ring of it into 

 the strengthening fibrous layer of the i)roboscis neck in Balano- 

 glossus, and the great development of the muscular coat of 

 the proboscis round the pro})oscis coelom, are all modifications 

 that seem to have i)roceeded from a nemertean to a balano- 

 glossoid type, on the plan indicated in Figs. 26 b and c. 



