. 451-452.] AMERICAN SOCIETY 



A Key to the Development of SipunculHs nubias': [,.hn II. 

 Gerould, Dartmouth Colle-e.— The (lc\ cloi^mcnt ol I'haso)!,.- 

 soma shows that the embryonal envelope or serosa of Sipmu iihis 

 midus, described by Hatchek, is a niodilied i)roiotroi h. In ihe 

 gastrula stage of each form the prototroeh tdiisisis ot twoor 

 three rows of broad, fiat cells which encircle the embryo, in 

 the trochophore of Ph. viilgare the aj^cal plate, tlie mid dor.sal 

 cord of ectoderm in the interru])tion of the prototi oc h and the 

 somatic plate are strikingly similar to tlie correspoiKhn- parts in 

 6". nudiis, but in the former no sinking beneath the vitelline 

 membrane occurs, and no amniotic cavities are formed, 'i he 

 resemblance between the trochoph(jres extends even to the ai)ical 

 rosette and to the cells which bear the postoral circlet of cilia. 



The remnants of the prototroeh of Sipunculus are cast off 

 with the vitelline membrane, the huge prototroeh cells of Phas- 

 colosoma degenerate during metamorphosis, and their substance 

 in the form of yolk granules passes backward into the newly 

 formed coelom. Thus the coelom of the larva becomes filled 

 with yolk granules, which Selenka mistook for blood corpuscles. 

 These gradually are absorbed. A cuticula is formed beneath 

 the vitelline membrane, the latter is shed, and an overgrowth ot 

 the region vacated by the prototroeh cells takes place. 



Sipunculids are primitive Trochozoa, closely allied to eha to 

 pods. The adult sipunculid retains the retractor muscles an<i 

 nephridia of the trochophore; and the loss of the prototroeli, 

 development of the coelom and elongation of the trunk are the 

 only fundamental changes which the trochophore undergoes ni 

 passing into the adult form. Resemblance of the troehophore 

 of Phascolosoma to that of chaetopods, c. g., Amphitrite, is 

 striking. Transitory metamerism of the mesoblastic bands and 



