NOTES ET REVUE xxvii 



sink away from the yolk membrane. The metamorphosis of trocli- 

 ophore into larva in Phascolosoma and Sipunculus is strikini^ly 

 similar as regards the growth of the somatic plate in the closure uf 

 the sides of the body, as well as in the casting olT of the yolk mem- 

 brane, but is curiousiy unlike in that the degenerating prototroch 

 cells in Sipunculus become the serosa and are cast ofT willi the yolk 

 membrane, whereas in Pliascolosoma their entire substance in the 

 form of yolk granules is passed into the cœlom. 



Larval DEVELOPMENT. No trace of prototroch remains in the 

 larva except the floating masses of yolk granules with which 

 the cœlomic fluid is filled (Fig. 7, vt.). Thèse graduallyare absorb- 

 ed and, by the end of the first week, the larva, in ail the three 

 species which I hâve studied, becomes transparent. Selenka des- 

 cribes this yolk in the cœlom as « Blutkorperchen », but 

 it is composed of non-nucleated spherules, whereas the true 

 blood corpuscles in small numbers can be distinguished in the 

 midst of it. 



During the third day, when the worm-like form is assumed 

 (Fig. 7), the larvœ sink to the bottom, where they continue to 

 twirl on their long axes [Ph. ruigare) until the postoral circlet of 

 cilia is lost. The young worm then creeps for a while by the ventral 

 cilia of the mouth région, and at the end of the first week is ready 

 to burrow in the muddy sand. 



The nephridia develop later than in Sipunculus. lu /'/(. Gouldii 

 they appear at about the sixty-fifth hour as ingrowths of ectoderm 

 on each side of the ventral nerve cord, in about the middle of the 

 body. This pair of solid ingrowths is covered with a layer of 

 mesoderm, which is a part of the cœlomic epilhelium ; a cavity 

 appears in the midst of the ectodermal rudiment, and nephroslorae 

 and nephridiopore break through. 



Epidermal organs (corpuscules sensitifs et glandulaires), wliicii 

 consist of oval clusters of ectoderm cells, that project slighlly into 

 the cœlom and cause small élévations of thecuticula above the gên- 

 erai surface, make their appearance at the âge of about live days. 



The simple prostomium of the larva grows out into two latéral 

 liât protubérances (Fig. 8 a), while simultaneously on the ventral 

 side a ciliated underlip is formed. Constant introversion and 

 thrusting forth of the head is a prominent characteristic ot the 

 larva. 



