192 



Introduction to Animal Morphology. 



(Sipunculus, Kef er stein). In Bonellia the eggs are 

 developed in a longitudinal, medio-ventral fold. 



The embryos are free-swimming, ciliated, with a 

 ciliary girdle and an anterior mouth dorsally over- 

 lapped by a bilobate upper lip ; ventrally, there is 

 either a single lower lip or several ciliated processes 

 (five in Phascolosoma minutum) the homologues of 

 the trochal discs of Rotifera. These in Sipunculus 





A, larva of Phascolosoma ; c, upper lip ; tt, lower lip ; /, intestine ; m, mouth ; 

 B and C, mollusc larvae. 



develop into the tentacles. Similar lobes are not 

 confined to Gephyrean larvae, but reach their greatest 

 development in the form Actinotrocha (probably the 

 larva of the Chaetopod Phoronis). The metamor- 

 phoses in development resemble those of the Echino- 

 dermal larvae, as only part of the larva is concerned 

 in the development of the adult. 



They are divisible into two orders : — 



I. Inermia — bristles and vascular system, none. Family 

 I, Sipunculidae — cylindrical, with the mouth at the tip of the 

 retractile proboscis surrounded with tentacles, often with 



