135 John H. Gerould, 



particularly true of Sipunculus. The embryonal envelop of this 

 form, or serosa, which I have shown (1903) to be simply a highly 

 modified prototroch, and therefore homologous to the velum of 

 Molluscs (cf. Meisenheimer, 1901), is cast off bodily during meta- 

 morphosis. 



The velum (using the term to apply also to cells adjacent to 

 the prototroch region proper) is similarly modified into a membrane 

 having a locomotor and protective function in the Solenogastres 

 (Dondersia and Frotonemia; Pruvot, 1890 and 1892) and in the 

 Lamellibranchs [Yoldia and Numla; Drew, 1899). This organ, like 

 the more typical velum of IscJmochiton (Heath, 1899), of Teredo 

 (SiGERFOOS, 1896) and of Dreisscnsia (Meisenheimer, 1901), and certain 

 Annelids {Polygordius, Wilson, 1890, CapiteUa, Eisig, 1898), is shed 

 during metamorphosis. 



In comparing the trochophore of Sipunculus (Hatschek, 1883) 

 with that of Chiton, as described by Kowalevsky (1883), one is 

 struck with the similarity between them as regards the invaginations 

 in the posterior lip of the stomodaeum. These infoldings lie in the 

 median plane of the embryo, at the posterior edge of the mouth 

 opening. The outer in each case is a larval glandular organ. In 

 Sipunculus it consists of an unpaired duct, at the free end of which 

 a pair of rounded glands are developed out of the epithelial lining 

 of the tubular invagination. The corresponding organ in Chiton is 

 a sac-like gland, which opens immediately behind the oral aperture. 

 Kowalevsky regards it as the rudiment of the pedal gland. The 

 latter in a heteropod {Firoloides, Fol, 1876) is bilobed, and appears 

 to resemble closelj^ the gland in a similar position in Sipunculus. 

 The inner invagination, situated behind the first, forms the radular 

 sac in Chiton, and corresponds exactly in position to the "Schlund- 

 kopf" in Sipimcidns. A vesicle of mesodermal origin and a pair of 

 clusters of gland cells become associated with the latter. The entire 

 organ entirely degenerates in the larva. Whatever may be the 

 function of the Schlundkopf of Sipimctdus, its ectodermal part corre- 

 sponds in form and position to the rudiment of the radular sac, and 

 the accessory glands of the stomodaeum to the pedal gland of 

 Molluscs. 



These facts, taken in connection with the general resemblances 

 of the trochophores, lead to the conclusion that the Sipunculids and 

 Molluscs are exceedingly closely related. It further appears that 

 forms which are generally regarded as primitive (Solenogastres, 



