98 ANIMAL FORMS 
cases (Sacculina) the sac-like body, attached to the abdo- 
men of crabs, is entirely devoid of appendages and any 
signs of segmentation. A root-like system of delicate fila- 
ments extends from the exposed part of the animal into 
the host and absorbs the necessary nutriment. The mouth 
and alimentary canal are accordingly absent—in fact, the 
body contains little but the reproductive organs and a very 
simple nervous system. 
98. Structure—In the internal organization of these 
smaller crustaceans many differences may be noted, though 
they are usually less profound than the external. Ordi- 
narily the alimentary canal is a straight tube passing 
through the body, and is provided with a pouch-like 
stomach, and a more or less clearly defined liver. In 
all, except the parasitic species, the external mouth-ap- 
pendages masticate the food, and in a very few of the 
above-described groups it may be further ground between 
the horny ridges on the stomach-walls. After this pre- 
liminary treatment it is subjected to the action of the 
digestive juices, and when liquefied is absorbed into the 
body. Here it is circulated by a blood-system of widely 
different character. In many cases definite arteries and 
veins are absent. The blood courses through the body in 
the spaces between the different organs propelled by the 
beating of the heart, which it is made to traverse. In 
Cyclops (Fig. 54) even the heart is absent, and the blood 
is made to circulate by contractions of the intestine. In 
most of these smaller Crustacea considerable oxygen is ab- 
sorbed through the body-wall; but in several species, for 
example, the fairy-shrimp (Fig. 53), special gills are devel- 
oped on the appendages of the body. 
99. Multiplication — Among the Crustacea thus far con- 
sidered the males are usually readily recognized owing to 
their small size. The females also are usually provided 
with brood-pouches in which the developing eggs are pro- 
tected. In almost every case the young are born in the 
