136 John H. Gerould, 



not only did not trace this vesicle back to its source, but he states 

 that it degenerates on the ventral side of the oesophagus of the 

 larva, at a stage when the intestinal blood vessel is already visible 

 on the dorsal and left sides of the alimentary tube. 



It is of course possible that Hatschek may have overlooked a 

 connection between these two structures, but it is pure assumption 

 to assume, in the face of our present information, that a homology 

 exists between the blood vascular cavity of the Sipunculids and 

 either the anterior body cavity of the Vermidea or that of the 

 Chaetopods. 



Whether Phoronis and the Bryozoa may have arisen somewhat 

 remotely from terebelloid ancestors does not concern us here. It is 

 sufficient to point out the undeniable fact that the embryology of 

 these animals shows that they are not closely related to the 

 Sipunculids. 



Phylogenetic conclusions that are based upon a single system 

 of organs, like the hydrostatic apparatus of the tentacles, without 

 taking into account all the available anatomical and embryological 

 facts that bear upon the problem under consideration, are of little 

 value. Thus, for example, there is apparently a marked resemblance 

 between the Molpadiidae and the Synaptidae. Both of these families 

 were formerly classed as Apoda, or footless holothurians, because 

 of the absence of ambulacra, the inconspicuous tentacles, and the 

 general form of the body; but a careful study of their entire 

 structure, and a comparison witli that of other holothurians, show 

 that these resemblances are a merely superficial adaptation to a life 

 of burrowing in the sand, and that the Molpadiidae are exceedingly 

 closely related to the Cticumariidae, a familj'^ with bushy tentacles 

 and with ambulacra, whereas the Synaptids stand by themselves, 

 remote from other holothurians, their nearest relatives being the 

 HoJothuriidae {Aspidochirotae). (See Ludwig, 1891, and Gekould, 

 1896.) 



Meyer's speculations, though interesting and suggestive, are, I 

 believe, not justified by the facts. The resemblance in the crown 

 of tentacles, the system of canals which are connected with them, 

 and the retractor muscles which cause their involution, are of super- 

 ficial character, and such as would be expected in diverse forms which 

 live under very similar conditions. Tentacles, and the canals that 

 are connected with them, are extraordinarily unstable, easily modified, 

 structui-es; and Meyee has taken no account of the multitude of 



