348 Ralph S. Lillic 



a marked tendency to become oriented with the anterior end direc- 

 ted away from the souree of illumination, and thus eventually, as 

 a result of the coutinued erawling- movements, to gather in regions 

 remote from the souree of light. The other marked peculiarity 

 which now first makes its appearance is a tendency to hring as 

 large a portion as possible of the hody-surface into contact with 

 solid bodies (positive stereotropism or thigmotaxis). Larvae freed 

 from adheriug solid particles, and placed in a vegsel containiug 

 loose finely divided débris of any kind, invariably exhibit restless 

 squirming and erawling movements, which continue until the body 

 is once more in contact on ali sides with solid particles. When 

 these particles consist of loose sand-grains, diatom-shells or similar 

 light, fiuely-divided material, the result is that the animai soon be- 

 comes enclosed in a rough tube foimed by the adhesion of such 

 particles to one another and to the body-surface by means of the 

 abundant secretion furnished by the ectoderma! glands. In this 

 manner are formed the rough tubes in which the remainder of the 

 larvai period is passed. When the larva is freed from such a tube, 

 the restless squirming movements are resumed, and continue until 

 another tube has been formed. Teudencies such as these, when com- 

 bined with the characteristic negative heliotropism, will, under natu- 

 rai conditions, plainly favor burrowing movements of the kind obser- 

 ved. l)Oth forms of reaction apparently persist until adult life, and 

 through them the characteristic behavior of the animai is largely 

 determined. 



After the dose of the swarming period and the assumption of 

 the burrowing habit of life, growth and elongation proceed steadily 

 and uniformly until the full number of somites bave been formed. 

 Elongation is due, as in other Annelid;i, to the activify of a poste- 

 rior growing zone, which occupies a position immediately anterior 

 to the originai situationof the paratroch (Figs. 7 — 10, Plates 22 and 23; 

 29 — 33, Plates 23 and 24). The large clear mesoderma! and ectoder- 

 ma! cells of this region contain large nucleolated nuclei, frcquently 

 found in process of mitosis, and continually give rise anteriorly to 

 new cells which constitute the basis of the newly forniing somites. 

 As each somite appears, it is divided oft" from the onc next succeed- 

 ing by a mesodermal septum; the growing zone thus retains un- 

 altered its distinctive characters, and occupies a Constant position rela- 

 tively to the posterior end. 



The short region posterior to the growing zone (post-mesodermal 



