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POPULAR SCIENCE REVIEW. 
nilly,” owing to the secretion from a pair of cement-glands in 
the neighbourhood of the stomach — the future ovaries, in 
fact — which runs along ducts to the antennal sucking-discs. 
In course of time the antennse atrophy and the animal becomes 
permanently fixed by the “ setting ” of the “ marine cement.” 
In the sessile Cirripedes the system of cement-glands is very 
complex, for at each period of growth new organs are formed, 
while the old glands are preserved, though now functionless, 
adherent to the basis of the shell. 
The homologies of the Cirripedes with the typical form of 
the Crustacean class, of which it is a member, will be indicated 
by comparison of figs. 6 and 5. Out of the twenty-one 
secondary aggregates, called segments or somites ” — to use 
the language of Herbert Spencer — which, distributed in sevens 
equally among the head, thorax, and abdomen, go to make up 
the perfect Crustacean, the adult Lepas possesses no less than 
seventeen, which are disposed thus : 
Peduncle (bearing a pair of eyes, a pair of antennse, and, in 
early Nauplius stage, an additional pair) =3 somites. 
Trophi of mouth (consisting of mandibles, maxillse, and 
outer maxillse, in pairs) = 3 somites. 
Thorax (bearing six pairs of cirri) = 6 somites. 
Eudimentary abdomen = 3 somites. 
But this only makes 15 as a total, and if the thoracic 
somites present are the six posterior segments, there must be two 
lacking between the outer maxillse and the first pair of thoracic 
legs. Now, luckily, these missing members will be found in 
Proteolepas, in which every segment in the body is “ as distinct 
as in an Annelid.” * 
There is a curious and aberrant group of Crustaceans, termed 
Rhizocephala, which live not only on, as harmless “ com- 
mensals ” — to use the language of Van Beneden — but by, and 
in their “ host,” generally some hermit- {Pagurus) or other 
genus of crab {Porcellanus), from whose abdomen they project 
like sausage-shaped excrescences filled with ova. Into such 
shapeless creatures the comparatively shapely Cirripede was 
converted, suggests Fritz Muller, by ‘‘ natural selection.” 
Now certain Cirripedes are in the habit, as has been previously 
stated, of fixing upon living animals. Suppose, then, at some 
primeval epoch, such an one had adhered to the soft abdomen 
of a hermit-crab, and that the cement-ducts in the peduncle, 
instead of keeping to the surface, had penetrated the body of 
* See further an article on “ The Lobster,” by Mr. St. George Mivart, 
F.E.S., in the Popular Science Eeview, Oct. 1868. 
