10 INVERTEBRATE ZOOLOGY. 



Type III. Ctenophora. 



Gastraea animals, all marine : structure a modified radiate, or doubly bi- 

 lateral one. Free-swimming ; moving by means of eight meridional rows of 

 peculiar organs derived from cilia. Sense organs at apical pole. Adhesive 

 cells with spiral, contractile thread developed in ectoderm, replacing the Cni- 

 doblasts of Type II. Middle layer a richly developed gelatinous tissue filled 

 with muscle cells. Germ cells probably from ectoderm (at present a matter 

 of controversy). 



[In comparing the three groups of Gastraea-animals, Types I-III, notice the difEerences 

 in development and use of the apical pole. In I it serves as an excurrent orifice ; in II as 

 point of support, and in III it is directed forward during locomotion and develops sense- 

 organs.] 



DIVISION II. HETERAXONIA {Coelomaia). 



Bilateral animals, primary axis either embryonic or suppressed. Secondary 

 (middle) layer from two sources ; (1) from paired epithelial sacks {^mesoderm): 

 (2) from wandering cells, derived from the primary endoderni(^»ie.se»f'/iy9?ia). 

 The first is a new formation and is (^ften reduced to single cells, or otherwise 

 obscurely indicated in the lower forms. The second is probably homologous 

 with the middle layer of the Protaxonia. Coelom present, either a proto- 

 coelom (derivative of Blastocoele) or metacoelom (cavities of the mesodermic 

 sacks). 



Type IV. Zygoneura. 



Animals derived from the hypothetical Trocliozoon (or an earlier form. Pro- 

 trochozoon) which are frequently repeated throughout the class in the 

 " Trochophore-larva " and similar forms. Nervous system consists of an api- 

 cal cerebral ganglion from which proceeds at least one pair of longitudinal 

 nerves. 



Sub-type I. Protonephridozoa {Scolecida). 



Entire body a " prosoma " (i. e. derived from the ancestral form), main body 

 cavity the protocoelom. Metacoelom represented as proton ephridium and pri- 

 mary sack-gonads. 



Cladus I. Platodes. 



Forms derived from the Protrochozoon, which is represented in a few mem- 

 bers of the group by a " protrochula " larva. The adult forms are flat, either 

 preeping or swimming slowly by means of cilia, or I'educed by parasitic life, 



