212 BULLETIN: MUSEUM OF COMPAEATIVE 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 

 thrown 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 larvae avoid the daylight and swim toward the least brightly 

 illuminated side of the aquarium. 1 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 larvae 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 larvae of Amarcecium also avoid the daylight, i. e. 

 are negatively phototactic ; but the larvae of Botryllus are strongly positive!// pho- 

 totactic, swarming toward ordinary daylight. This difference may perhaps be 

 explained by the difference in habitat of the parent organisms. Botryllus, whose 

 larvae seek the light, is commonly found in w r ell illuminated places, e. g. adhering 

 to floating eel-grass. On the other hand, Ciona and Amaroecium, whose larvae 

 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. 



