264 



CffiLENTERATA. 



Up to the present time, the nervous system of the Ctenophora 

 (fig. 203) is but imperfectly known. There is no doubt that the 

 large vesicle found at the aboral pole, with its clear fluid and 

 vibratile otoliths, is a sense-organ ; it is also exceedingly probable, 

 taking into consideration the organization of the Acalepha, that 

 the central nervous system of the CteiwpJtwa is contained in the 

 thickened base of the vesicle, t/te Otolith j>/<tf?, especially as the latter 

 is also closely united with a second sense-organ, the sagittal polar 

 areas, which have already been described by Fol as olfactory organs, 

 and is also directly connected with the swimming plates by eight 

 ciliated grooves. 



True neniatocysts are but seldom found in the ectoderm of the 



Ctenophora, but they are represented by 

 peculiar fixing or prehensile cells, the base 

 of which is prolonged into a spirally coiled 

 thread, while the projecting and convex free 

 end (tig. 204) is of a glutinous consistence, 

 and becomes readily attached to any object 

 which touches it. 



The Ctenophora are hermaphrodite. Both 

 kinds of generative products arise on the wall 

 of the vessels of the swimming plates or of 

 blind sac-like diverticula of the same. Some- 

 times their production is localised (Cesium) ; 

 sometimes they originate along the whole 

 length of the canals, one side of the latter 

 being beset with egg-follicles, the other with 

 sperm-sacs (Beroe). The germ layers, which 

 arise from the ectoderm, are covered by 

 entodermal epithelium, and are separated 

 from one another by a projecting fold. 

 Ova and spermatozoa pass into the gastro- 

 vascular cavity, and are ejected through the apertures of the 

 same. 



The fertilized ovum, which is enclosed by a loosely fitting 

 membrane, consists, as in the case of many Medusce, of a thin outer 

 layer of finely granular protoplasm (exoplasm) and a central food 

 yolk (endoplasm), containing vacuoles. The segmentation, which is 

 complete, leads to the formation of two, four, eight segmentation 

 spheres, each of which, like the original ovum, consists of a central 

 mass, surrounded by a thin layer of finely granular protoplasm. In 



PIG. 204. Smooth muscle 

 fibres, prehensile cells 

 (kf), and tactile cells ((>), 

 from the lateral filaments 

 of the tentacle of E/i/ilo- 

 camig stationis (after R. 

 Hertwig). i-f, Prolonga- 

 tion of the contractile 

 thread of a prehensile cell. 



