DEVELOPMENT OF GEPHTBEAN8. 203 



that of the Annulata. There is in Phascolosoma no true 

 ovary, but the eggs float in masses in the capacious body- 

 cavity, the animal being a hermaphrodite. 



Phoronis is from the highly developed crown of long, 

 slender tentacles, and its complicated blood-system, remark- 

 ably like the Serpulm, with which Annelids it is by some 

 authors associated. The alimentary tube, however, is like 

 that of Phascolosoma, the intestine folded and ending next 

 to the mouth. No nervous system has been detected. A 

 pulsating artery is attached to the upper side of the long 

 oesophagus, and its branches go into the tentacles from an 

 oesophageal ring. " Two venous trunks open from the sin- 

 uses above and behind the arterial branches, and then pro- 

 ceed downwards, half encircling the oesophagus, till they 

 unite in a large vessel on its neural surface." (Dyster.) 

 This worm is minute, about four millimetres in length, and 

 lives in a tube buried in holes in rocks. It has a strong re- 

 semblance to a Polyzoon, but connects the Gephyrea with 

 the true Annelids. 



In the Sipunculus-like worm Phascolosoma, and in Pho- 

 ronis, there is a well-marked metamorphosis, and the larvae 

 are somewhat like those of Annelids. The larva of Phas- 

 colosoma is cylindrical, the head small, with a circle of cilia, 

 but there are no arms as in the larva of the Phoronis. 



The earliest observed stage of Phoronis * is a free-swim- 

 ming larva, the body transparent, ciliated, with an umbrella- 

 like expansion on the head, covermg the region of the mouth, 

 while the end of the body is truncated. At this stage it is a 

 true Gephahila, like that of Echinoderms and worms. Af- 

 terwards four projections arise at the end of the body, and 

 twelve long, arm-like projections grow out, the larval form 

 now being fully attained. In tTiis condition it was de- 

 scribed as a mature animal under the name Actinotrocha. 



When the Actinotrocha is about to transform into a Pho- 

 ronis the end of the intestine bends up, opening outward 



* In our Outlines of Comparative Embryology this account of the 

 metamorphosis of Phoronis is by mistake regarded as descriptive of 

 Sipunculus on pp. 157, 158, under Development. The word Phoronis 

 on th"se pages should be substituted for Sipunculus. 



