212 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
Allusion has been made to the rapidity of development of the egg. 
Within twelve hours after fertilization the larval form is attained, the 
tail being coiled round the trunk within the egg membranes. Hatching 
usually occurs within the next twelve hours, i. e. in the first night after 
the laying of the eggs. It is brought about by twitchings of the larval 
tail, which finally rupture the egg membranes. Under certain con- 
ditions the larva does not succeed in breaking through the egg mem- 
branes.  Metamorphosis then sets in almost immediately, and is 
completed within the egg membranes, a functionally free-swimming 
stage being wholly suppressed. This is regularly the case in Molgula 
Manhattensis, where hatching of the larva is exceptional, the new, meta- 
morphosed individual arising just where the egg settled after it was 
tbrown out into the water and fertilized. However, in Ciona the more 
primitive course of events is usually pursued. The larva then escapes 
from the egg membranes as a miniature tadpole, the “test cells” clinging 
to its thin and adherent covering of homogeneous, non-cellular mantle 
substance secreted by the ectoderm. These test cells are soon brushed 
off as the tadpole swims about ; they have no connection, as is now well 
known, with the cells to be found later in the mantle of the adult. 
The larve avoid the daylight and swim toward the least brightly 
illuminated side of the aquarium.! Here they attach themselves, usually 
near the surface of the water, to the side of the aquarium. Sometimes 
the attachment is by the head end, as it is commonly said to be, but I 
have more often observed the larvee attached by the sticky mantle sub- 
stance at the tip of the tail, the body then hanging head downward 
against the side of the aquarium. 
The larval stage varies in duration from twenty-four hours to several 
days. It is terminated by the beginning of metamorphosis, whose suc- 
cessive steps are well known through the description of Kowalevsky (66 
and '92), Willey (93), and others. 
1 I have observed that the larvæ of Amarocium also avoid the daylight, i. e. 
are negatively phototactic ; but the larvae of Botryllus are strongly positively pho- 
totactic, swarming toward ordinary daylight. This difference may perhaps be 
explained by the difference in habitat of the parent organisms. Botryllus, whose 
larva seek the light, is commonly found in well illuminated places, e. g. adhering 
to floating eel-grass. On the other hand, Ciona and Amaracium, whose larve 
avoid the light, more often occur in darkened places, the former on the under side 
of stones, the latter adhering to piles underneath wharves, or on the sea bottom in 
sheltered spots near shore. 
