METAMORPHOSIS OF ACTINOTROCHA. 215 
A little consideration shows that the metamorphosis 
itself is in reality a sudden and extreme dorsal flexure of 
the body, which may be considered as the ontogenetic 
repetition of a habit of adult ancestral forms. It differs 
from the latter, however, in the immediate disappearance 
of external signs of the flexure, while in the phylogeny 
this disappearance probably took place very gradually. 
Fig. 13 shows the larval body bent at right angles towards 
the dorsal side. 
A few further remarks may be added in regard to the 
systematic position of Phoronis. Its very striking resem- 
blance to a simple hippocrepian Polyzoon is entirely second- 
ary, and is a very curious illustration of the acquisition of 
almost exactly similar structures to meet similar conditions 
by organisms having no near genetic relation. As already 
stated, the Polyzoon-like character is a result of strictly 
tubicolous life; and the latter, again, may depend in part 
upon the absence of structures, like the complex sete of 
tubicolous Annelides, adapted to effect the ready protrusion 
or withdrawal of the body. Schneider’s comparison of 
Actinotrocha to the Polyzoon larva Cyphonautes is purely 
superficial, and affords no proof whatever of relationship 
between the Polyzoa and Gephyrea. Further, his notion of 
the relation of Phoronis to its larva, which he compares to 
that existing between the polyzoon polypide and the cystid 
from which it develops, is manifestly based on a misunder- 
standing of the real nature of the metamorphosis, for the 
adult cannot be said, in any sense, to arise by a process of 
gemmation upon the larva. 
The significance of the life-history of Phoronis becomes 
apparent when we pass to a consideration of some of the 
remarkable metamorphoses undergone by other animals. 
As already pointed out, these metamorphoses are in most 
cases of so complex a character that it is impossible to 
follow out in imagination their past history as circum- 
stantially as we are able to follow the history of Phoronis. 
Nevertheless, we can trace the same principle in many of 
these cases, and in a few instances a somewhat similar 
process. For example,in the well-known and very remark- 
able case of Pilidium the animal is subjected, during its 
development, to two entirely different environments. During 
the earlier part of its life it is, like Actinotrocha, an active 
free-swimming animal. At a later period it leads an utterly 
different life, burrowing in mud or sand, living upon dif- 
ferent food with new enemies, and subjected to many other 
new influences. Correlated with these great differences of 
