308 MALACOZOA. APODA. 



and respiratory apertures above. They are however 

 generally fixed by attachment to other bodies, and being 

 thus incapable of locomotion, are destitute of any organ 

 analagous to the muscular foot or arms of the other 

 Malacozoa ; for which reason, and to preserve a uni- 

 formity of nomenclature, I have named them Apoda or 

 Footless. Some of them, however, are free, and move 

 by the contractions of their body or its covering. They 

 may be defined as animals having two mantles or tunics, 

 of which the outer sometimes forms a tube open at both 

 ends, but is generally a closed sac, of a leathery or carti- 

 laginous consistence. Within this external sac is a delicate, 

 soft internal tunic or mantle, enclosing the body, or the 

 various organs. This inner mass is not in contact with 

 the inner walls of the outer sac, there being between them 

 a space filled with water. In the upper part of the outer 

 sac are two apertures, leading by two canals into the 

 interior of the inner sac. Of these apertures, the 

 upper leads into a cavity, on the inner surface of which 

 the branchiae are expanded in the form of a net-work. 

 The water which enters carries with it the particles of 

 nutritious matter, which are received by the mouth, 

 placed at the lower part of the branchial cavity, and pre- 

 senting a simple aperture, without any labial or lingual 

 appendages; the stomach is generally large, the intes- 

 tine wide and curved in the form of the letter s, and 

 terminates in the lower of the two external apertures. 

 There is a liver of large size, and the genital organs, 

 similar to those of the Malacozoa Lamellibranchiata, 

 terminate in the anal aperture. The branchiae vary in 

 form, but are always small, and not divided into laminae. 

 These animals are all marine, most of them living affixed 



