380 INVEETEBEATA CHAP. 



coelomic origin, from which the internal funnel or nephrostome is 

 formed. 



As the body grows longer and longer the anus appears to move 

 forwards, but this appearance is simply due to the fact that the part 

 of the body intervening between the apical plate and the anus does 

 not grow nearly so fast as the portion situated behind the mouth, 

 on the ventral surface ; this disparity of growth is the essential 

 characteristic of all Podaxonia. 



If we review the development which has just been described, we 

 shall find ourselves driven to the conclusion that, not only are 

 Phascolosoma and its allies descended from the common Ctenoph ore- 

 like ancestor of Annelida and Mollusca, but that they have diverged 

 from the Annelid stem after the beginnings of segmentation had been 

 acquired, and that they represent one mode in which the descendants 

 of the primitive Annelida were adapted to a burrowing life. 



SIPUNCULUS 



The development of the well-known Mediterranean genus 

 Sipunculus has also been worked out by Hatschek (1883) though 

 not at all in the same detail as Gerould has worked out Phascolosoma. 

 It agrees in all essentials of its embryonic and larval history with 

 Phascolosoma, the chief differences being the form which the proto- 

 troch assumes and the mode of disposing of it. 



In Sipunculus the prototroch, instead of being represented by 

 sixteen primary prototrochal cells, is represented by a broad mantle 

 of comparatively small cells carrying short cilia. This mantle is, 

 however, incomplete in the mid-dorsal line, where a narrow line of 

 sunken cells connects the head- and trunk-blastema. This line of 

 cells corresponds to the dorsal cord of Phascolosoma. When the 

 larva metamorphoses the whole of the mantle is cast off, as in Nucula 

 amongst Mollusca, and is not absorbed into the coelom as in 

 Phascolosoma. 



PHORONIDEA 



A form of great interest, which was placed in the old group of 

 Gephyrea under the division Gephyrea tubicola, is Phoronis. 

 Phoronis is now made the type of a special family, the Phoronidea, 

 which Lankester considers to belong to the Polyzoa, but which we 

 regard as more nearly related to the Sipunculoidea whose develop- 

 ment we have just discussed. 



Phoronis agrees with the typical Podaxonia in the ventral 

 development of the body, but instead of living in sand and mud it 

 inhabits a leathery tube which it secretes for itself. It possesses, like 

 most Podaxonia, a curved lateral extension of the lips of the mouth 

 bearing ciliated tentacles, but these, instead of being prae-oral as in 

 Phascolosoma, are post-oral. 



