4^ Introduction to Animal iMorphology. 



mata, while in the Echinodermata, Chsetognatha, and 

 Enteropneusta, as well as in the higher sub-kingdoms, 

 a second or adult mouth becomes substituted for the 

 embryonic : these are called Deuterostomata. Possibly 

 the Deuterostome worms should form a separate sub- 

 kingdom, but I have refrained from dividing the great 

 group Vermes until we have fuller information regard- 

 ing some of its classes (Gephyrea, Rotatoria, and 

 many of the aberrant forms). 



The Deuterostomata differ primarily according to 

 the nature of the body cavity, which maybe formed : — 

 I St. By outgrowths from the gastrula, as in Echino- 

 dermata, Chsetognatha, and Enteropneusta (hence 

 called Enterocoela). 2nd. By the splitting of the 

 mesoblast, as in Molluscs and Polychaete Annelids, 

 (hence called Schizocoela), or by an invagination 

 of the epiblast, as in Tunicata and Amphioxus, 

 (hence called Epiccela). Something like this method 

 will doubtless be the classification of the future, but 

 much embryological observation is yet required to 

 correct and improve it. 



CHAPTER VIII. 



PROTOZOA [GoldfllSs). 



Plastides, homo- or heteroplasts, generally minute ; 

 with no nervous system nor internal cavity ; often 

 forming shells or spicules, and containing, in their 

 protoplasm, granules, usually a nucleus, often a 

 nucleolus, and a clear contractile vesicle. They move 

 by pseudopodia or cilia, rarely by a contractile layer. 



