The Development of Phascolosoma. 79 



1. Introduction. 



The observations on which this paper is based were made upon 

 the living- eg-gs and young of three species of this Sipunculid, and 

 supplemented by the study of a large amount of preserved material. 

 The two forms to which I have given especial attention are Pkasco- 

 losonia gouldii (Diesing) ^) of the American coast and Fli. vulgare 

 (Blainv.) of the English Channel. While studying the latter species 

 -at the Laboratoire Lacaze-Duthiees at Roscotf in Finistère, I gave 

 some attention also to the development of Ph. elongatum (Keferst.), 

 the eggs and larvae of which differ in many points from those 

 described by Selekka (1875) for this species, so that I am inclined 

 to the opinion that the form which he identified as Ph. elongatum 

 at Villefranche is a different species, or variety, from that which 

 was originally described by Keferstein (1863) as occurring in the 

 British Channel. 



Wherever in this paper the species upon which an observation 

 is made is not expressly mentioned, the statement may be understood 

 to apply equally to both Ph. vulgare and Ph. gouldii. In Ph. gouldii 

 I have studied the maturation of the (i^g and early stages of 

 cleavage, and have followed the development of the trochophores 

 and larvae to the age of a month. Ph. vulgare, how^ever, is by far 

 the most favorable for embryological research, of any of the three 

 species that have come under my observation. Its large translucent 

 ^^g^, when laid, can be artificially fertilized with no difficulty, and, 

 if the eggs are taken from the body cavity during the breeding 

 season, a considérable number of the maturer oocytes can usually 

 be fertilized. I have accordingly made use of this species to repeat 

 and extend my observations on the maturation and fertilization, to 

 study the cleavage, and to follow the development of the trochophore 

 through its metamorphosis, and that of the larva up to the age of 

 six weeks. 



Many of the figures which illustrate this paper were drawn ' 

 during the summer of 1894 at Mr. Alexander Agassiz' laboratory 

 at Newport, R. I., where, a year previous to that, I had made a few 

 preliminary observations. Publication has been deferred through my 

 desire to give as complete as possible an account of the life history 



1) See Appendix A. 



