Structure and Development of the Nephridia. 345 



consideration. Child (1900) has given an accurate account of the 

 early development within the eg-g-membrane. My own descriptions 

 will have reference to the metamorphosis and larvai development 

 up to a period at which most of the detìuitive external characte- 

 ristics are complete. 



Very little care is needed iu order to preserve the larvae for 

 weeks or even months in a healtby condition, and capable of 

 growth and development. The egg-strings are placed in clean sea- 

 watcr in large, tìat, well-lighted dishes covered by sheets of glass 

 to prevent excess of evaporation, and contaiuing a few pieces of 

 Ulva for aeration. It is advisable to change the sea-water at inter- 

 vals of a week or ten days; otherwise the dishes require very little 

 attention. The organic debris present, derived from minute Algae, 

 particles of decaying Ulva, and the bodies of dead larvae, seem to 

 fiiruish the larvae with suftìcient food for the development of a 

 fairly large proportion of their number. Under these conditions, 

 larvae have beeu kept in the laboratory for periods of 14 to 15 

 weeks, apparently iu a perfectly healthy condition and exhibiting 

 ali the normal activities. Development seems, however, to progress 

 more slowly than under the naturai conditions, for the largest S])eci- 

 mens reared never exceeded a length of 15 mm., although exhibiting 

 iu other respects au almost perfect agreement with the adult in 

 appearance, anatomica! structure, and behavior. . The food-supply is 

 in ali probability insufficient for rapid growth; this is indicated by 

 the fact that if kept in sea-water to which Carmine powder has 

 been added the larvae usually exhibit a greatly increased rate of 

 growth, especially iu the early stages. It would no doubt be pos- 

 sible, by the employment of suitable methods of feeding, to rear 

 them to more advanced stages than the above. 



The larvae leave the egg-strings in from two to three days after 

 oviposition, in the form of slightly elongated maggot-like free-swim- 

 miug orgauisms (about 0.3 mm. in leugth), which exhibit a most 

 pronounced positive phototaxis combined with negative geotaxis. As 

 a result of these tendencies they swim rapidly to the light side of 

 the dish and there gather iu enormous numbers at the surface of 

 the water. At this stage a larva possesses in addition to the peri- 

 stominm which is without setae), three setigerous trunk somites, in 

 each of which are two paired sets of setae corresponding to the 

 notopodial and neuiopodial setae of the adult. The notopodial setae 

 of each side are generali}- two in number, spoon-shaped and spear- 



23* 



