86 



I had already come to the conclusion that the larva I had 

 studied and the adult described by Strethill Wright were the 

 same species from a comparison of the latter with the newly meta- 

 morphosed young. Harmer gives the specific characters of 

 Phoronis ovalis as follows : — Size, reaching at least 6 mm., the 

 transverse diameter about 0-25 to 0-35 mm.; lophophore oval, 

 much broader than long, and indented as usual posteriorly ; number 

 of tentacles about 22 ; metasome sharply divided into an upper 

 region with strong longitudinal bundles of muscles and a lower 

 region with an extremely thin body-wall in which muscles are 

 absent or at most very sUghtly developed : about fourteen bundles 

 of longitudinal muscles occur on each side of the upper part 

 of the body. The newly metamorphosed young proceeding 

 from ActinotrocJia branchiata has the characters : — Size, about 

 3 mm., the transverse diameter about 0-2 mm.; lophophore oval 

 and indented as in P. ovalis ; number of tentacles about 22-26 ; 

 metasome divided into two regions, the upper presenting well- 

 developed longitudinal muscles and the lower being thin-walled, 

 but in this case the lov/er division is roughly about a third of the 

 upper ; about fourteen bundles of longitudinal muscles on each 

 side in the upper part of the metasome. There can be Uttle doubt 

 therefore that the agreement between the young Phoronis and the 

 adult as defined by Harmer is sufficient to prove that they are 

 the same species. Incidentally, the small degree of development 

 of the lower part of the metasome in the young as compared with 

 the adult serves to show that growth takes place by an extension 

 of the lower end of the metasome, and is thus directly continuous 

 with that which gives rise to the invaginated metasomal wall of 

 the larva. 



The larva (Fig. 1) is one of the most graceful which comes 

 under the eye of the plankton investigator. It is transparent, 

 but pigment usually adds to its elegance. Spots of pigment are 

 dotted round the edge of the hood, and on the anal ring, usually 

 two spots are present on each of the tentacles towards their bases 

 and isolated flecks may frequently be seen on the posterior part 

 of the bod}^, and even on the wall of the intestine. The pigment 

 is bright green viewed bj^ reflected light, dark green in transmitted 

 light, and is soluble in alcohol. The larva is liberated when it 

 has about two pairs of tentacles, and the tentacles are added to 



