Vol. 5, p. 243 7 246. Junk 13, 1927. 



Occasional Papers 



OF THE 



Boston Society of Natural History. 



THE VALIDITY OF MOLGULA ROBUST A (VAN 



NAME) AS A SPECIES DISTINCT FROM MOLGULA 



MANHATTENSIS (DE KAY). 



BY ALFRED M. LUCAS. 



The validity of Molgula robusta (Van Name) as a species 

 distinct from M. manhattensis (De Kay) is questioned by Hart- 

 meyer (1923). He maintains the former is only a variety of 

 M. manhattensis. The purpose of this paper is to record observa- 

 tions that establish M. robusta as a valid species. 



During the summer of 1925, while I was working at Woods 

 Hole, Mass., a number of simple ascidians dredged from Vineyard 

 Sound were brought into the laboratory. One of these was 

 identified as M. robusta (Van Name) . A study of its embryology 

 revealed the following facts. 



The eggs of M. robusta are usually shed at night, probably 

 toward morning. With the eggs and surrounding them is a 

 mucous mass, very viscous, which tends to hold them together in 

 strings. When these strings of eggs and mucus are stirred in the 

 sea water, considerable time is required for the mass to settle to 

 the bottom, a condition not true for the eggs when free from the 

 mucus. It may be that in nature the current, carrying the 

 strings of mucus, is a means of disseminating the eggs. Eggs 

 isolated from the mucus are not adhesive. 



The eggs in their development during the first two days show 

 very few obvious changes for the reason that the deep reddish- 

 purple pigment renders them opaque. In two and a half days 

 the epidermal tubes, five in number, push out from the body, but 

 as yet they do not cause an evagination of the tunic. A day later, 

 one of these tubes projects beyond the tunic, and by means of 

 this epidermal tube the embryo becomes attached to the object 

 upon which it is resting, provided attachment has not already 

 been effected by the mucus. The embryos were observed from 

 day to day in their development, and at no time was either a free- 

 swimming larval stage or a statolith or similiar organ noted. 



The development of M . robusta was compared with that of M. 



