MUSEUM OF COMPARATIVE ZOOLOGY. 179 



between Annelids and Sipunculids, as has long been maintained by 

 Hatschek ('80 and '83) on embryological grounds. 



The existence of at least giant fibres has been proved for Echiurus by 

 the researches of Greeff ('79) and Spengel ('80, p. 487), and more recently 

 for Thalassema by Eietsch ('86, p. 402), so that the presence of corre- 

 sponding ganglionic cells may be reasonably assumed. This is, then, a 

 further ground for separating the Sipunculids from the Echiurids, aud 

 for assigning to the latter a closer relationship to the Annelids than the 

 former have. This position has been defended from an embryological 

 standpoint by Hatschek ('80, p. 71) and Conn ('86, p. 399). 



In spite of the well known conservatism of the nervous system, I am 

 vfell aware of the dangers of such conclusions based upon the study of a 

 single system or a single form. The foregoing comparison is offered, then, 

 merely as a new side light on the unsettled question of the position of 

 the Sipunculids, and in the hope that the accumulation of evidence from 

 various sources may some day bring a clear and full solution of the 

 problem. 



January 20, 1891. 



Addendum. 



During the correction of the proof-sheets there has appeared a second paper 

 by Shipley ('91) on Phymosoma (P. Weldonii, n. s.). It is interesting to note 

 that the gland cells there described (p. 114) correspond very closely to the 

 multicellular glands of S. nudus, except that no connection with nerve fibres is 

 reported. Shipley affirms positively (p. 11.5) "the absence of those skeletal 

 cells which formed so interesting a feature" of P. varians (Shipley, '90, p. 9). 

 That such a tissue does not exist in S. nudus has already been emphasized. 

 This is then strong proof that it is an individual peculiarity of the one species, 

 rather than an ancestral relic. In general the claimed relationship of Sipuncu- 

 lids and Phoronis seems to me to have little in its favor beyond the external 

 similarity of the two forms. 



It is a pleasure to see that Shipley and I have both arrived independently at 

 the same conclusions regarding the vascular system. He ('91, p. 116) does not 

 regard it as important in respiration, and explains the csecal diverticula of the 

 dorsal vessel, which might be looked upon as strengthening the view of its 

 respiratory nature, as merely reservoirs for the increased overflow from the 

 tentacles, which are exceptionally numerous in this species. 



