218 THE ANATOMY OF INVERTEBRATED ANIMALS. 



is the so-called Actinotrocha.^ 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 JPhoronis^ 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 MdUer, is the embryo of Sternaspis. 

 The affinities of the Gephyrea with the Turhellaria, with 

 the Annelida^ and with the Rotifera^ are unmistakable. In 

 fact, it may be doubted whether &ternaspis should not be 

 associated with the JPolychceta, and JBonellia is in manj^ 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 8ip%inculu8 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 * (Fig. 56, II., S). 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 Metamorplaose der Actinotrocha hrancUata." 

 (" Archiv fur Anatomie," 1862.) 



2 " Ueber die Larve des Sipunculus nudus.''^ (" Archiv fiir Anatomie," 

 1851.) 



