112 PROCEEDINGS OF THE ACADEMY OF [Feb., 



animal. Such an explanation is certainly permissible, but it would 

 be worth while first to inquire whether a corresponding structure 

 exists in any of those groups to wdiich Dinophilus can be consideretl 

 related, and from which it may have derived this structural pecviliarity. 

 Such a group is found in the annelids, whose larva, the trochophore, 

 represents either an ancestral condition, or else a stage interpolated in 

 the ontogeny very early in the racial history. In this larva, thanks 

 to the researches of Kleinenberg (1886), confirmed by Meyer (1901). 

 it is known that at an early stage there is present, embedded in the 

 ectoderm, beneath the prototroch, a circular cord of nerve fibres, con- 

 sidered by Kleinenberg as the primitive nervous system of the larva, 

 and homologous with the nervous system of the medusa. At a late 

 period in the development, during the metamorphosis of the larva 

 into the segmented worm, this nerve-ring is crossed, on the future 

 ventral surface of the worm, by the circumoesophageal commissures, 

 forming a structure shaped like the letter H (see Kleinenberg's fig. 31, 

 Taf. IX). This intercalated segment of the nerve-ring persists for a 

 long period, but finally suffers complete absorption and disappears. 

 Now, one of the chief functions of the nerve-ring is that of innervating 

 the prototroch, the principal organ of locomotion of the larva, and of 

 controlling its motions. If, then, the prototroch should persist, it 

 would be ciuite probable that at least a portion of the nerve-ring would 

 persist with it, and it would ciuite likely be that portion which persists 

 longest in the ontogeny. This, I believe, is what has happened in 

 Dinophilus. Moreover, there are also indications of the persistence of 

 the lateral portions of the nerve-ring (see fig. 1). When it is remem- 

 bered that the second preoral ciliated band is in all probability homo- 

 logous with the prototroch of the annelid larva, it will be apparent that 

 there are good reasons for considering the preoral transverse commis- 

 sure of Dinophilus as the persisting remnant of the nerve-ring of the 

 trochophore larva of the annelids, preserved not merely by force of 

 inheritance — since a multitude of examples might be cited to show that 

 this is not of itself sufficient to save an organ or structure from dis- 

 appearance — but rather by the physiological needs of the animal. 



The oesophageal or stomatogastric nerve is not only found in D. 

 gigas (Weldon, 1886), D. tceniatus (Harmer, 1889a), and D. vorticoidcs 

 (Schimkewitsch, 1895), but in annelids generally, as, for example, 

 Oligognathus (Spengel, 1881) and Saccocirrus (Fraipont, 1884). 



The nerve innervating the ventro-lateral longitudinal muscles has 

 not yet been described in any species of Dinophilus; but renewed 

 research of other forms might bring it to light. Its close connection 



