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 



