The porifera and coelentera . ; nucleated jwrtiou forms part ofthe epidennal mos;iic uii the free surface of the l>o<ly.(From Lankester, after Herlwij;.) lature of the bell and the velum ; by their agency the contraction and con-sequent progression of the bell are chiefly eflected. The tentaclesare highly contractile, and are provided with strong longitudinalmuscles. Sensori/ cells, which are elongated and columnar, and areprovided with palpocils, are well developed at the bases of the THE HYDROMEDUSAE tentacles. Subepithelial ganglion cells and nerve Jibrillae form a scattered plexus in
The porifera and coelentera . ; nucleated jwrtiou forms part ofthe epidennal mos;iic uii the free surface of the l>o<ly.(From Lankester, after Herlwij;.) lature of the bell and the velum ; by their agency the contraction and con-sequent progression of the bell are chiefly eflected. The tentaclesare highly contractile, and are provided with strong longitudinalmuscles. Sensori/ cells, which are elongated and columnar, and areprovided with palpocils, are well developed at the bases of the THE HYDROMEDUSAE tentacles. Subepithelial ganglion cells and nerve Jibrillae form a scattered plexus in the ectoderm in connection with sens§ry and muscle cells, especially on the subumbrella; they are concentrated at the lip of the bell into a nerve ring, which is divided by the insertion of the velum into outer and inner portions, connected by nerve fibrils through the mesogloea. Connected with thenerve ring are the senseo/r/a/i« (Hertwig, 19; Eimer,20) or special aggregationsof sense cells. They arereferable to four chief. Fio. lOh.—Scattered nerve-ganglion cells from of Aurdiu. (From Lunkister, after types. 1. Orelli or eye spots 1^^^^are generally found at the bases of tentacles. In their simplest form they consist of a fewsense cells between which are scattered a few pigment cells ; in theircomplete development, the sensory and pigment cells are gioupedinto a definite organ of subspherical shape (Fig. 11), which projectsabove the general surface, and may secrete a cuticular lens (Lizzia).The whole structure is ectodermal. 2. Otoci/sts are found under two chief forms : (a) in the simplerof these the organ consists of an open subumbral pit at the baseof the velum, the cells of which secrete each an otolith of organicand calcareous nature (Mitrocoma); (h) in the more complex typethe pit becomes converted into a closed vesicle, containing one ormore otolithic cells, which are usually supported on whole structure is ectodermal, and may occ
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Keywords: ., bookcentury1900, bookdecade1900, booksubje, booksubjectctenophora
