The Development of Phascolosoma. 137 



anatomical and embryological facts which tend to separate the 

 Sipunculids from the Vermidea proper and from the Echinoderms. 



Conclusions. 



The superficial resemblance of the larva of Fh. gouldii, im- 

 mediately after metamorphosis, to a rhabdocoelous turbellarian led 

 me at first to consider the possibility of a close relationship between 

 the Sipunculids and the Turbellaria, but after a more precise 

 study of the metamorphosis and of the larval development, I abandoned 

 the idea. 



From the comparisons that have here been made the conclusion 

 has been reached that the Sipunculids are either degenerate Annelids, 

 primitive Annelids, or primitive monomeric Trochozoa that are 

 somewhat more closely related to the Annelids than to the Molluscs. 

 The question as to the relationship of the Sipunculids therefore 

 resolves itself thus : (1) are they degenerate or primitive forms, (2) if 

 primitive, should they be classed as Annelids, or as an independent 

 phylum of the Trochozoa? 



(1) The Sipunculids, like PoJygordius, have an unpaired and un- 

 segmented ventral nerve cord, which I believe is to be regarded as 

 the sign of primitive structure. Another primitive character is the 

 retention of the retractor muscles of the trochophore by the adult 

 Sipunculid. These muscles are of such general occurrence in the 

 trochophore of the marine Annelids and Molluscs, that their reten- 

 tion in later life in the Sipunculids indicates clearly, it seems to 

 me, the primitive nature of this group. In brief, the entire organi- 

 zation of an adult Sipunculid is exceedingly simple and trocho- 

 phore-like. Not only the simple nerve cord and the retractor 

 muscles of the trochophore, but also the single pair of thoracic 

 nephridia, are retained in the adult. 



It may be urged, however, that the probable absence of the 

 head kidney in Sipunculus and in Phascolosoma is an argument 

 against regarding these forms a primitive. As is well known, this 

 organ is found in PoJygordius, in the Chaetopods generally, and in 

 EcMurus, but among the Molluscs it is known to occur only in the 

 Lamellibranchs and a few Gastropods. It has not been observed in 

 the primitive Soleno g astres, the Chi ton es, nor in Dentalmm. 

 In this state of our knowledge, or ignorance, we are therefore 

 hardly justified in assuming that this nephridium is necessarily 

 found in all primitive Trochozoa. 



