1880. | Zoölogy. 895 
passes into the adult condition through the development of a 
deep pouch-like invagination of the ventral body-wall which be- 
comes attached to the stomach, and is at length suddenly evagi- 
nated, dragging out into its cavity a long U-shaped loop of the 
intestinal canal, and thus producing the remarkable flexure of the 
latter in the adult. This pouch after its evagination forms the 
greater part of the body; the opposite or dorsal side of the larva 
becomes much shortened, and is only represented in the adult by 
the short interval between the mouth and the anus. A study of 
two species of Actinotrocha occurring in Chesapeake Bay (to be 
elsewhere more fully described), suggests the following «xplana- 
tion of the significance and origin of this strange metamorphosis: 
Considerations which for want of space cannot be here detailed, 
leave little doubt that the primitive forms among the Gephyrea 
are those which, like Thalassema or Bonellia, have the mouth 
and anus at opposite extremities of the body. Forms like Pho- 
ronis or Phascolosoma, in which these two openings are near the 
same extremity, are evidently derivative; in the case of Phoronis, 
at least, I assume this structure to have been brought about by 
the flexure of a primitive form into a U-shape (in order, perhaps, 
to void excrement through the mouth of the tube inhabited by 
the worm), and the subsequent obliteration of external evidences 
of this flexure through coalescence of the two parts of the body 
thus flexed. Such a habit of flexure may be actually observed 
among some Polycheta and Holothurians; and in the latter case 
several stages in the obliteration of flexure by coalescence may be 
observed. The Polychztous larva, Mitraria, affords a further 
illustration of this point. 
Through whatever process we assume the peculiar flexure of 
the intestine to have been effected, it is clear that the pouch of 
Actinotrocha, doth before and after its evagination, is a develop- 
ment of the ventral region of the body. And it follows that in 
the adult the ventral region is greatly in excess of the dorsal, 
while in the larva they are externally nearly equal. The pouch 
of the larva is evidently a provision to admit of extensive increase 
in the ventral region as a preparation for the adult structure, with- 
out changing the external form of the body, and thus without im- 
pairing the adaptation of the larva to its Pelagic life. Thus the 
creature is enabled to pass at once, by a single leap, as it were, 
‘om one set of conditions to an entirely different set, without 
having to pass through intermediate stages. Evidently a great 
Saving of time and energy is thus effected. : 
pouch is probably to be regarded as a specialization of a 
primitive simple infolding of the ventral body-wall. he meta- 
morphosis is in reality a sudden and extreme flexure of the larval 
body, and may be considered as the ontogenetic repetition of a 
habit of adult ancestral forms.—Admuna B. Wilson. 
