58 COMPARATIVE ANATOMY CHAP. 



Comprises all the remaining races of the animal kingdom, viz. the fourth, 

 Vermes ; the fifth, Arthropoda ; the sixth, Mollusca ; the seventh, Echinodermata ; 

 the eighth, Tunicata ; and the ninth, Vertebrata. 



THE SECOND KACE OE PHYLUM OF THE ANIMAL KINGDOM. 



ZOOPHYTA OR CCELENTERATA. 

 Systematic Review. 



CLASS I. Gastrseadse. Without pores in the body wall and without tentacles. 

 ,, II. Porifera or Sponges. With pores in the body wall, without tentacles. 

 ,, III. Cnidaria or Stinging Animals. Without pores in the body wall, with 



tentacles. 



Of the three chief classes of the Ccelenterata, the Gastrceadce show essentially the 

 structure of a Gastrula, while the other two groups contain animals more highly 

 differentiated, which, developing in entirely different directions, cannot be com- 

 prehended in one description. 



CLASS I. 



The Gastrseadse. 

 Systematic Review. 



A. The Physemaria, Haliphysema, Gastrophysema. 

 B.. The Dicyemidse. Dicyema (Fig. 53). 

 C. The Orthonectidse. Rhopalura, (Fig. 54). 

 Appendage : Trichoplax adhcerens (Fig. 55). 



The Gastrceadce are animals whose structure essentially corresponds 

 with that of the Gastrula. In some forms the organisation is com- 

 plicated ; in others, no doubt in adaptation to the parasitic manner of 

 life, somewhat simplified. The Physemaria are bi-laminar tubes attached 

 to the sea bottom by that portion of their bodies which is opposite to 

 the aperture. The ectoderm consists of fused cells (syncytium) ; the 

 endoderm of collar cells, each with a flagellum. The sexual products 

 are developed in the endoderm. Foreign bodies are contained in the 

 ectoderm. Were the body wall of the Physemaria perforated by pores 

 they would have to be considered as the simplest sponges. 



The bodies of the Dicyemidce (Fig. 53) and Orthonectidce (Fig. 54) 

 which are parasitic in Cephalopoda, Echinodermata, and Turbellaria, also 

 consist of two principal layers; the ciliated ectoderm forms an un- 

 broken layer of not very numerous cells round the inner solid layer, 

 which is generally considered to be endoderm, and this layer consists 

 either (Orthonectidce) of a mass of cells, or of one single multinuclear 

 axial cell (Dicyemidce). The oral opening and gastral cavity have here 

 disappeared in the same way as in the Cestoda. The body of the 

 Orthonectidce is outwardly ringed, and between ectoderm and endoderm 

 has a layer of ectodermal muscular fibres. In the Orthonectidce 

 spermatozoa and eggs are produced in the endoderm, but in different 



