-142 - On the Evolution of the Vertebrata, [February, 
sessile mode of life. Such adaptation is often displayed in a very 
special modification of parts, as in the anterior limbs of some of 
the parasitic Crustacea; in the mouth parts of some Arachnida ; 
in the feet of the sloth, and in the jaws of the ant-eaters. 
Embryology has furnished, and will furnish, many important 
hints and demonstrations as to the true meaning of the rudimen- 
tary condition or absence of parts, and thus indicate the phylogen- 
etic connections of animals. Thus the origin of the Tunicata 
from primitive vertebrate-like forms would probably never have 
been suspected but for embryological studies; and the origin of 
the very peculiar order of Rotifera has been explained in like 
manner, But embryology has its limitations, for the transitional 
characters presented by embryos are only partially of the nature 
of a record of the structures which belonged to their ancestors in 
successive geological ages, and are frequently special adaptations 
to the necessities of their embryonic life. Such are the stato- 
blasts which are present in fresh-water sponges and Polyzoa, and 
wanting in the marine forms; and the allantois and placenta of 
Vertebrata, In a number of groups the embryo seems to have 
been more susceptible to the influence of the environment than 
the adults) It results that in many cases the phylogeny can only 
1A remarkable instance of this state of things appears in the history of the evolu- 
tion of the insects, It is quite impossible to understand this history without believ- 
ing that the larval and pupal states of the highest insects are the results of a process 
of degeneracy which has affected the middle periods of growth but not the mature 
results. The earliest insects are the Orthoptera, which have active aggressive larve 
and pup, undergoing the least changes in their metamorphosis oT and 
never getting beyond the primitive mandibulate condition at the 
morphosis of the jawed Neuroptera is little more marked, and tis are one of the 
oldest orders. 
The highest orders bite ne undergo a marked PASEO (Coleoptera, Hy- 
menoptera), the Hymen even requiring artificial intervention in some in- 
stances to make it a Finally the most specialized e the suctorial Dip- 
tera and Lepidoptera, especially the latter, present us with peti unprotected more or 
or 
poses of reproduction. As is well known, many imagines (Saturniidæ, Œstridæ 
can perform no other function, and soon die, while in “some Diptera the incomplete 
larvee themselves reproduce, so that the metamorphosis is never completed. 
This =- is parallel to that proposed by Dohrn to account for the origin of the 
` Ammoccetes larval stage of the Marsipobranchii. He supposes this form to be more 
degenerate a its probable ancestral type in the ancestral line of the Vertebrata, as 
it is inferior to its own adult. An inactive life in mud is supposed by Dohrn to have 
been the effective cause, An inactive life on the leaves of plants, or in dead car- 
cases, has probably been the cause of the same phenomenon in the Lepidoptera and 
Diptera. 
