250 INVEUTEBRATE MORPHOLOGY. 



strong cilia. Behind the mouth are a number of ciliated 

 tentacle-like processes arranged in a horseshoe-shaped 

 curve, their cilia, together with those of the edge of the 

 prostomial hood, forming a band encircling the mouth. The 

 digestive tract opens to the exterior at the posterior ex- 

 tremity of the body, and the axis of the body is the axis 

 passing through the anus and the centre of the prostomial 

 lobe. A little later {B) an invagination {in) of the body-wall 

 into the coelom of the larva develops on the ventral surface 

 behind the band of ciliated processes and becomes of a con- 

 siderable size. At the time of the metamorphosis this in- 

 vagination is suddenly everted (Fig. 113, C and D), the intes- 

 tine being carried with it as a loop, and entirely new axial 

 relations are thus brought about. The long axis of the body 

 is now {D) almost at right angles to what it was in the 

 Actinotrocha, and since the invagination originally formed 

 on the ventral surface of the larva, the body of the adult 

 Fhoronis must be regarded as formed by an excessive de- 

 velopment of the ventral surface, the dorsal surface being 

 represented only by the short interval between the mouth, or 

 rather the epistome, and the anus. The epistome represents 

 the prostomial lobe of the larva, and the ciliated processes 

 represent the lophophoric region, though they themselves are 

 afterwards replaced by the permanent tentacles. 



There can of course be no question but that this remarkable metamor- 

 phosis is a secondary phenomenon, and it seems probable that its acquisi- 

 tion stands in relation to the tubicolous habits of the adult which neces- 

 sitate the change of the principal axis of the body. The metamorphosis is 

 the means of avoiding a slow and tedious change necessitated by the differ- 

 ent habits of the larva and the adult, just as the occurrence of the chry- 

 salis stage in the development of the butterfly is required on account of 

 the great differences between the mouth-parts of the larval caterpillar and 

 the adult butterfly. 



The affinities of PJioronis cannot be considered to be finally settled as 

 yet, though there has been a tendency of late years to associate them with 

 the Polyzoa. They also seem to show affinities to the Gephyrea, and by 

 some authors are considered more correctly referable to that group. The 

 tendency to develop the ventral surface of the body at the expense of the 

 dorsal and so to form a new body-axis is seen in Sipunculus and carried 

 to its culmination in Phoronis, and further similarities between the two 

 forms are to be found in the character of the nephridia and in the occur- 



