PHYSIOLOGY OF THE INVERTEBRATA. 77 



In Appcdicularia there is a caudal appendage which 

 contains a notochord ; but in the Ascidians the caudal 

 appendage is only present in the larval condition of the 

 animal — a condition closely resembling the tadpole or larval 

 frog. In fact, these animals are degenerated Vertebrates.* 

 Another point in which the Ascidians approach the Vcrtcbrata 

 is that the pharynx is also a respiratory cavity. 



We have now come to the end of our chapter on Inverte- 

 brate digestion in general. Below the Actinid the alimentary 

 canal communicates with the body cavity, but with the excep- 

 tion of the Ti'nicata,in all those forms higher (than the Actinia) 

 in the animal scale, the body cavity and alimentary canal are 

 entirely separated from each other. 



In regard to the masticatory apparatuses ; (l) a gizzard 

 is observed in the Rotifcra, Olirjochccta, the higher Inmda., 

 Polyzoa, Gasteropoda, and Cephalopoda. (2) The com- 

 plex masticatory apparatus of Echinus, with its five jaws, 

 each traversed by a tooth, is probably nothing more 

 than altered epithelium which has become hardened. (3) 

 Hardening of the membrane of the buccal mass is a 

 further advance in the apparatus designed for mastica- 

 tion ; as these structures are at the commencement of the 

 alimentary canal, and are separated from the stomach. In 

 the Anndida {cj., Zumh-icns and Hirudo) the so-called jaws 

 are hardened parts of the epithelium of the mouth. These 

 structures may be functionally compared to the teeth of the 

 Vcrtehrata. (4) The Ai'thropoda present a highly developed 

 masticatory apparatus in the jaws, which are appendages of 

 the body segments. " The simplest condition is met with 

 in the Mijriaiwda, as the centipede. A small labrum above 

 the oral aperture, a pair of mandibles or hard crushing jaws, 

 a labium below the oral aperture, with side lobes. The 

 Arachnida (spider and scorpion) have labrum and mandibles, 



the Chalhnffer lieports, 1S82, part i. p. 49; 1S86, part ii. pp. 22, 52 and 88 ; 

 part iii. pp. 22 and 42. 



* Sec Ray Lankester's book, De(jeneration, p. 41. 



