216 



/ A' VER TEBRA TE ZOOLOG Y 



as a place for attachment. When they become attached to the 

 bodies of other animals, it is a very common thing for them to 

 become dependent upon the animal which offers shelter, and as a 

 consequence there is here shown in great detail the development 

 of the parasitic habit. Various species become more or less 

 dependent upon whales and sharks, while Sacculina becomes 

 attached to the abdomen of decapod crabs and undergoes a para- 

 sitic degeneration rarely equaled. All traces of appendages 

 and of digestive organs are lost, and, as the name implies. 



Ter^um 



Adductor 

 Muscle-- 



Liver- 

 Oviducf-- 



Tesf-f's ■ 



Cemenigland 



-Penis 



--Carina 



- Vas deferens 

 Ovar^ 



Fig. 100.— Barnacle. 



Sacculina becomes a mere saclike structure with a rootlike 

 system penetrating the body of the host for absorbing food. The 

 relationships of this degenerate form are shown only through the 

 larval stages which are fully characteristic of the barnacles. 



Subclass Malacostraca 



The Malacostraca are frequently spoken of as the higher 

 Crustacea. Lobsters, crayfishes, crabs, shrimps, prawns, sow 

 bugs, scuds, and sand fleas are common names of some represen- 

 tatives of this subclass. The body segments, so highly variable 

 in number in members of the foregoing groups, are in the Mala- 

 costraca almost rigidly limited to twenty or twenty-one. Usuallj^ 

 the head consists of a prostomium and five additional somites, 

 the thorax of eight, and the abdomen of seven of which the 

 last (the telson) bears no appendages. 



