116 G. HERBERT FOWLER. 
these two groups. Though it is customary to assume that de- 
generacy is a necessary consequence of parasitism, few more 
favorable conditions for the preservation of ancestral charac- 
teristics could be found than those of an animal which adopted, 
at an early period of biological time, a semi-parasitic (com- 
mensal) habit upon hosts exposed to no special dangers,' and 
would therefore not be subjected to any violent struggle for 
existence, provided always that some means for reaching its 
host be retained for at least the duration of larval life. Such 
conditions are realised by the Ascothoracida: a free larval 
existence (judging from the development of Laura) brings them 
either to an Antipatharian or a Bathyactis, both well protected 
by skeleton and nematocysts, and in the latter instance at 
a depth where the struggle does not seem to be severe. 
The agreement of the Ascothoracida with the Lepadide (the 
least modified of Cirrhipedia) is very strongly marked. While 
it is impossible to present a definition of the latter group which 
shall include all its characteristics, it will yet be admitted that 
the presence of only one pair of antennz in the adult, of an 
indifferently-segmented thorax bearing six pairs of appendages 
(biramose in Synagoga), of a rudimentary abdomen, on the 
ventral side of which springs an enormous penis, of an oviducal 
opening at the base of the first thoracic limb (Hoek, Cirrhi- 
pedia [anatomical part], ‘Chall. Rep. Zool.” vol. x), and 
the position of the testes in the bases of the thoracic appen- 
dages, are all features of prime morphological importance, 
which characterise the Lepadide and are shared among the 
Ascothoracida. The presence of shell-pieces on the mantle of 
the one, and the reduction of the mouth parts and simplifica- 
tion of the appendages in the other, are characters of secondary 
value, dependent on their different modes of life. 
While the points of resemblance to the Cirrhipedia are so 
strong and so numerous, the presence of a bivalve carapace, 
which is not a mere lamina composed mainly of epidermic 
structures, but a capacious lateral outgrowth of the 
1 If the host be modified in accordance with changing or unfavorable con- 
ditions, modifications will probably be also induced in the parasite. 
