The Development of Phascolosoma. ]^J7 



throughout this period twirls on its long-itudinal axis, usually near 

 the bottom of the aquarium. This motion, which is due to the 

 action of the postoral circlet, continues till the larva is about five 

 days old, when the postoral cilia are lost, and the larva, like Ph. 

 gouldii at an earlier period, creeps by the action of the ventral 

 prostomial cilia. In other respects the two species keep equal pace 

 in their development. 



The larvae of both species at this time (50—60 hours) are 

 about 0.26 mm in length' in an ordinary degree of extension, though 

 individuals of Ph. vulgare may extend themselves to a length of 

 0.33 mm, as shown in Fig. 54. The general shape is cylindrical 

 but the trunk has become expanded with the yolk from the proto- 

 troch, which has passed back into the coelom, and now exceeds the 

 introvert in transverse diameter, as much as the prototrochal region 

 in the trochophore surpassed the trunk. In other words, the relative 

 proportions of the trunk and prosoma have become inverted by the 

 displacement of yolk. The floating particles of yolk in the coelom 

 of the living animal, which are kept in circulation by its constant 

 movements, render the body opaque, so that the enteric tube can 

 be distinguished only with difficulty. 



Two sorts of coelomic corpuscles are found in the larvae. The 

 larger are identical with the nuclei of the prototroch. They agree 

 in size and in number with these nuclei, which at the close of 

 metamorphosis are cast into the coelom. They are surrounded some- 

 times by a homogeneous remnant of cytoplasm, as in Fig. 80a, 

 sometimes only by their nuclear membrane. The nuclear contents 

 have undergone only slight changes. They consist of a deeply-staining 

 chromatic reticulum, in the midst of which is usually a nucleolus 

 and a large unstainable vacuole. 



These prominent nuclear corpuscles are found in larvae of all 

 ages up to at least a fortnight. During all this time they undergo 

 no change, unless some divide by amitosis. The normal number of 

 these corpuscles (nineteen) was seen, however, in individuals of 

 fourteen days. I have no information as to their fate. Their size 

 corresponds closely with that of the amoebocytes of the adult, and 

 is about half that of the flat blood corpuscles (Fig. 81). 



The smaller coelomic corpuscles are of less constant size 

 (Fig. 80b, c). It is probable that they are derived from the coelomic 

 epithelium at the posterior end of the body cavitj'. The largest of 

 them are found in the younger larvae (80 hours to 7 days). They 



