218 THE ANATOMY OF INVERTEBRATED ANIMALS. 



is the so-called Actbiotrocha. 1 An invagination of the ven- 

 tral integument of the larva connects itself with the middle 

 of the intestine, and then, becoming evaginated, pulls the in- 

 testine, in the form of a loop, into the ventral process thus 

 formed, which gives rise to the body of the Phoronis, while 

 the tentacles of the larva grow into those of the adult. 

 Schneider has suggested that the bell-shaped larva, with long 

 setse, termed Mitraria by Mailer, is the embryo of Sternasjyis. 

 The affinities of the Gephyrea with the Turbellaria, with 

 the Annelida, and with the Rotifer a, are unmistakable. In 

 fact, it may be doubted whether Sternaspis should not be 

 associated with the Polychata, and Ponellia is in many re- 

 spects comparable to a colossal Rotifer. Their usually as- 

 sumed connection with the Echinodermata is more question- 

 able. The circular canal which communicates with the cavi- 

 ties of the tentacles in Sipunculus has been compared to the 

 ambulacral system of the Echinoderms, but the manner of 

 its development is not yet sufficiently understood to justify 

 the expression of an opinion on this subject. Krohn has de- 

 scribed a bilobed organ on the ventral face of the gullet of the 

 larva of Sipunculus, which opens externally in front of the 

 ciliated band by a narrow ciliated duct a (Fig. 56, II., 3). It 

 has a striking similarity to the " water-vessel " of the larva 

 of Balanoglossus, which, however, lies on the opposite side 

 of the body. 



1 " Schneider, " Ueber die Metamorphose der Adinotrocha Irancliiata." 

 (" Archiv fur Anatomie," 1862.) 



2 " Ueber die Larve des Sipunculus nudus." (" Archiv fur Anatomie," 

 1851.) 



