The Development of Phascolosoma. 135 



definitive hydrocoel of Ecliinoderms is supposed to be developed from 

 the cavity of the left of these pouches of the diapln-agm. 



It is of course generally recognized that in Phoronis and the 

 Bryozoa a diaphragm divides the coelom into an anterior chamber, 

 or cavity of the lophophore, and a posterior chamber, containing 

 the reproductive elements. Meyer believes that similar conditions 

 are found in the Sipunculids, and assumes that their so-called vas- 

 cular cavity is equivalent to the cavity of the lophophore. Now it 

 is well known that a striking resemblance exists between the so- 

 called blood-vascular system of the Sipunculids and the water vas- 

 cular system of the holothurians , particularly the Synaptids. In 

 both a circular canal is found, which communicates in front witli 

 the tentacular cavities and behind with diverticula (Polian vesicles 

 of holothurians, intestinal sinuses of Sipunculids). Delage et Hérouard 

 (1897, p. 15) and Cuénot (1900, p. 398) have commented upon this 

 resemblance, and have expressed the opinion that it is due to a 

 secondary adaptation to a life of burrowing in the sand. 



However this may be, Meyer follows, in imagination, the evo- 

 lution of terebelloid stock into the Sipunculids, by degeneration 

 and loss of metamerism, and the development from the latter of 

 Phoronis and the Brj^ozoa. Another branch of the same Annelid 

 stock develops along similar lines until the sipunculid form is 

 nearly reached, when it becomes modified into the ancestor of the 

 Ecliinoderms, 



The scope of this paper does not permit a full account of 

 Meyer's views. Several questions are suggested by them. The one 

 which is of most immediate interest is this: is there embryological 

 evidence tending to show that the blood vascular system of the 

 Sipunculids corresponds either to the anterior body cavity of Chaeto- 

 pods, or to the cavity of the lophophore of their Vermidean relatives? 



As regards the origin of the blood vascular system of Sipun- 

 culids absolutely nothing is known. I have not seen any rudiment 

 of it in the larvae of Phascolosoma , and it probably appears very 

 late in the larval, or post-larval, development. Nor have I found 

 any trace of a pair of anterior mesoblastic vesicles, such as Meyer's 

 hypothesis demands. Meyer, however, makes the interesting guess 

 that the mesodermic vesicle, which Hatschek (1883) found in connec- 

 tion with the pharynx (Schlundkopf) of the larva of Sipunculus, 

 may be a median, united, pair of anterior coelomic vesicles, and the 

 rudiment of the future blood vascular system. Hatschek, however, 



