MUSEUM OF COMPARATIVE ZOOLOGY. 163 



Numerous facts have been adduced by Andrews ('90, p. 419) to prove 

 the branchial nature of the tentacles in S. Gouldii, chiefly the circulation 

 and the red coloring matter of the corpuscles. Certain structural and 

 other peculiarities compel me to deny the respiratory nature of this 

 system in S. nudus. As was pointed out by Brandt ('70, p. 23), the 

 extreme thickness of the layer of connective tissue in the tentacles 

 would militate against the opinion that respiration takes place to any 

 considerable extent in this part. Furthermore, although I have watched 

 S. nudus in aquaria for considerable periods of time, not only when they 

 were lying upon glass, but also when they were on the surface of the 

 sand, and in their burrows wherever these were adjacent to the glass so 

 as to permit observation, I have seen the tentacles extruded but seldom, 

 and never for more than a second or two, until the water had become so 

 impure as to partially narcotize the animals. The respiratory value of 

 the tentacles when retracted cannot be regarded as very important ! 



But the greatest objection to assigning a respiratory character to this 

 system would seem to be the utter inadequacy of the internal mediation 

 between the vessels and the ccelomic fluid. The possible importance of 

 this system in a respiratory direction must be seriously questioned when 

 one considers that the ring canal and the two blind sacs (in S. Gouldii 

 but one !) buried in the connective tissue of the oesophagus, which at 

 best expose but one half their surface to the ccelomic liquid, are the 

 only means of transmitting oxygen from the so-called vascular system 

 to the general body fluid. The observations of Keferstein ('62, p. 47) 

 upon living animals — these were made on Phascolosoma elongatum of a 

 few millimeters in length and fully transparent — showed a constant move- 

 ment of the fluid, but no passage of it from the vessels into the tentacles, 

 or vice versa, except under considerable pressure or violent injuries. 



The probable lack of respiratory function in the vascular system can- 

 not be extended to all Sipunculids. In this connection it is of great 

 interest to notice that various species are provided (Selenka, '83, p. xix) 

 with several or many branched lateral appendages attached to the blind 

 sac. Such organs are found in Phascolosoma Semperi, P. maniceps, 

 Phymosoma asser, Dendrostoma signifer, et al. All of these forms pos- 

 sess, according to the same author, long thin filamentous tentacles 

 (cf. his generic descriptions and figures). This peculiarity suggests at 

 once the probability of a respiratory nature for the tentacles ; and its 

 occurrence in single species of various genera would indicate that it is a 

 secondarily acquired function. 1 



1 See Addendum. 



