IV 



PHYLUM CCELENTERATA 



cubical ; afterwards they become drawn out so as to be more spindle-shaped, and 

 at the same time become bent in such a way as to present a concave internal and 

 a convex external surface ; the latter is always covered with long cilia. The 

 two most posterior are usually semi-cylindrical and surround the posterior end 

 of the endoderm cell. 



The endoderm cell is either almost completely cylindrical or spindle-shaped. 

 It is covered in its entire extent by ectoderm cells. There is a differentiated 

 cortical layer, beneath which the finely giannlar gelatinous contents are at first 

 homogeneous, but afterwards become vacuolated. In the middle of the cell is a 



Km. ir,-_>._ Dicyema paradoxum, 

 with infusorifonn embryos. (From 

 ' 



FKJ. lt>3. Dicyema paradoxum, 

 witli vermiform embryos. (From 

 Uronn's TIiii.i-i-t.ii-li.) 



large oval or ellipsoidal nucleus. In addition to the nucleus of the endoderni 

 cell itself other nuclei also occur in it ; these are the nuclei of the i/trw-a {/<. 



Two forms of adults are to be distinguished, termed respectively the 

 the iiematogene and the rhombor/eite : the former (Fig. 163) give rise to r> rm/form, 

 latter (Fig. 16'2) to infusoriform embryos. The rhombogenes are shorter and thicker 

 than the nematogenes ; the number of ectoderm cells is smaller, and the germ- 

 cells small and produced endogenously in the axial cell, instead of being formed 

 in special cells enclosed in the axial. The rhombogene and iiematogene forms 

 may be phases in the life-history of the same individual ; but some always 

 possess the iiematogene form. 



The first-formed germ-cells are derived by cell-division, accompanied by 

 mitosis, from the nucleus of the axial cell. Subsequently they increase in 

 numbers by division. A germ destined to give rise to one of the vermiform 



