220 



not the trunk divided into thorax and abdomen ; which have 

 from twenty-four to sixty pairs of feet, and even more, but 

 which never have wings : the spiders (Fig. 174), which have 

 not the head distinct from the thorax, which have always 



Shell.Mantle.Tentacula. Mouth. Nerves. Muscles 



Foot. 



Intestine. 



Stomach. 



Branchite. 



Mantle. 



Anterior 

 Ganglions. 



Respiratory Tubes. 

 Fig. 183. Anatomy of an Acephalous Mollusc (Mactra). 



only four pairs of limbs or feet, and which respire the air like 

 all the preceding, but which have no tracheae, and receive the 

 fluid into pulmonary pouches : the Crustacea (Fig. 173), which 



