124 INVERTEBRATE MORPHOLOGY. 



of the latter extend throughout the entire thickness of the 

 mesoglcea and are inserted by their branched extremities 

 into the ectoderm on the one side and the endoderm or 

 stomodseal ectoderm on the other. They are contractile in 

 function, consisting of a central protoplasmic axis containing 

 a nucleus and of a peripheral contractile substance. In addi- 

 tion to these there are other much finer fibres which have 

 been supposed to be nervous, and on the outer surface of the 

 mesoglcBa, between it and the bases of the ectoderm-cells, is 

 a network of stellate ganglion-cells whose processes overlap 

 but do not unite with each other. They are especially abun- 

 dant in the region of the meridional rows of plates ; just as 

 the slender fibres of the mesoglcsa are especially abundant 

 below the aboral sense-organ and the meridional grooves. 

 Further information is, however, required as to the nervous 

 system of the Ctenophores. 



The class may be divided into two orders : 



1. Order Tentaculata. 



The members of this order possess tentacles either with 

 or without the primary axis. The simple forms, such as 

 Pleurobraohia, belong to this order, as well as the lobate and 

 ribbon-shaped forms. In the lobate forms, such as Bolina 

 (Fig. 66) and Mnemiopsis, there is at each end of the sagittal 

 axis a large lobe developed into which four of the meridional 

 canals are continued ; two of the canals, those nearest the ex- 

 tremities of the transverse axis, pass around the edge of the 

 lobe and unite with each other, while the other two, which 

 also unite, are thrown into arabesque-like twistings. The 

 Venus'-girdle, Gestum, is ribbon-shaped, being flattened in the 

 transverse plane and much drawn out in the sagittal plane ; 

 the result being the great extension of four of the meridional 

 plate-rows and the almost complete disappearance of the 

 other four. In its young stages, however, Cestum is a spher- 

 ical form closely resembling the simple genus Mertensia. 



