Aug., 1891. 
ANIMAL PEDIGREES. 
169 
ANIMAL PEDIGREES. 
BY A. MILNES MARSHALL, M.A., M.D., D.SC., F.R.S., 
BEYER PROFESSOR OF ZOOLOGY IN OWENS COLLEGE, MANCHESTER. 
(Concluded from page 152.) 
An excellent illustration of this downhill progress is 
afforded by the Rhizocephala, a curious group of parasitic 
crustaceans, of which the genus Sacculina is perhaps the 
best known member. 
The adult Sacculina, Fig. 14, is found as a soft, shapeless 
bag. an inch or so in length, attached to the under surface of 
the tail of a crab by a fleshy stalk, which, passing through the 
skin of the crab, spreads out within it into a complicated 
system of branching tubular roots by which the parasite sucks 
up the juices of the crab, on which it depends for food 
As regards structure, the Sacculina in its fully developed 
form is little more than a bag of eggs enclosed in a loosely 
fitting outer skin, with a single orifice through which the 
young escape. A nervous system is present, but there are no 
traces of limbs, digestive system, heart, breathing organs, or 
sense organs. Indeed, examination, however careful, of an 
adult Sacculina would fail to afford any clue as to its real 
zoological affinities. 
Development, however, in obedience to the potent law of 
recapitulation, shows us at once that Sacculina is a Crustacean, 
more closely allied to the Barnacles than to any other of the 
more familiar members of the group. From each of the 
exceedingly numerous eggs which a Sacculina produces, there 
emerges a minute, free swimming larva of' the type known as a 
Nauplius, Fig. 11, characterised by possessing a somewhat 
pyriform body, a single median eye, and three pairs of 
swimming appendages or legs. Nauplius larvae are widely 
spread amongst Crustaceans. All Entomostraca, except the 
Cladocerans, hatch in this form, and Nauplius larvae are found 
in individual members in nearly all the higher groups as well. 
The only special peculiarity about the Sacculina Nauplius is 
that it has neither mouth nor digestive organs of any kind, 
these being rendered unneccessary by the presence of a 
considerable quantity of food yolk. The Sacculina larva 
continues its free existence for a time; it casts its skin, or 
rather its cuticle, many times, emerging each time of rather 
more complicated structure, though of actually smaller size, 
for it cannot yet take in food. Ultimately it reaches the 
condition shown in Fig. 12, and spoken of as the pupa stage. 
The pupa is enclosed in a bivalved carapace, very similar 
to that of a Cypris. It possesses a pair of well-developed 
