The Development of Phascolosoma. 105 



hours by the growth ventrad of the somatic plate and the in- 

 vagination of the descendants of the stoniatoblasts. 



7. Development of the Trochophore (24 — 48 hours). 



The trochophore is spherical until about the thirty-sixth hour, 

 when the body becomes more sharply convex at the posterior 

 extremity, giving evidence of the more rapid growth at that pole, 

 and a perceptible difference between the antero-posterior and trans- 

 verse axes is for the first time noticeable. Thus it assumes an 

 ovoid, or top-shaped, form (Fig. 49, 50, 60, 62), the ectoderm of 

 which consists of three distinct parts, the apical plate, prototroch 

 and trunk. 



The trochophores during the second day, when the pigment 

 spots appear, begin to be strongly positively phototactic to diffuse 

 sunlight and even to the direct rays of the sun. The embryos of 

 Fli. vulgare usually keep close to the surface during this period, 

 twirling on their longitudinal axes and describing at the same time 

 a spiral course through the water. The trochophore of Ph. gouldii, 

 however, rises only a little above the bottom of the aquarium, and 

 I am doubtful as to its being pelagic. As soon as Fh. vulgare begins 

 to show a marked elongation of the trunk (43 — 44 hours), it evinces 

 a marked tendency to sink to the bottom of the aquarium, but the 

 rays of the setting sun falling directly upon the trochophores at that 

 time bring them to the surface, where they congregate in the more 

 brightly illuminated regions. As darkness sets in they become 

 evenly distributed through the aquarium, but when brought into a 

 lighted room they rise again to the surface, and gather near the 

 edge next the lamp. The sinking beneath the surface is significant 

 of internal changes which accompany the metamorphosis into the 

 larva that is to follow. 



A cuticula in the process of formation is visible during this 

 period beneath the striated yolk membrane (Fig. 50, 59, 60, 86—89). 

 In its incipient stages it is much less highly refractive than the 

 latter, and is distinctly granular. 



The apical plate (Fig. 46, 47, 50, 86—89) consists of a circular 

 mass of small cells, surrounding a group of four large cells which 

 I have called the definitive rosette. The latter bear sensory flagella. 

 The rosette occupies the centre of the apical plate in the young 

 trochophore ^Fig. 47), but at a later stage (45—48 hours) it lies 



