DETERMINATION OF SPECIMENS. 57 



XXX Pelagic, tadpole-shaped 

 animals, with long, flat- 

 tened tail; body with in- 

 current and excurrent 

 orifices. 



Ascidiacea, sub Preverte- 

 brata. 



XXXX Minute, transparent pe- 

 lagic organisms; possess 

 ing bands of cilia. [Here 

 belong the larval forms of 

 Echinoderms, Nemerteans, 

 Enteropneusta, some An- 

 nelids and other marine 

 Invertebrates.] 



Note. — Nearly every important group possesses members which live as para- 

 sites either upon or within the bodies of other animals, and these, especially when 

 in the latter position, often become so much modified that the usual characteristics 

 of the groups to which they belong no longer apply to them. In some cases these 

 modifications are so great that the real affinities may be learned only by a careful 

 study of the embryological development, in some stage of which the animals are apt 

 to resemble their free-living allies. Of these modifications the most usual are a 

 reduction or entire loss of the locomotive organs and those of special sense, and the 

 acquirement of suckers, hooks, and other adhesive organs ; aside from this the gen- 

 eral shape becomes vermiform or sac-like. It is thus impracticable to attempt to 

 include the more reduced parasitic forms in the above key, and although many of 

 them may be found and their position determined by its aid, it will be necessary in 

 other cases to consult some more special authority. As this key is intended, how- 

 ever, for the beginning student only, and as the study of these parasitic forms be- 

 longs to those more advanced in the science, it will doubtless prove a sufficient aid 

 to tiiose for which it is designed. 



