PROBLEMATICAL SENSE-ORGANS. 61 



umbrella and the sub-umbrella are not separated by an cndoderm- 

 lined space (atrium or canals), they are by a solid layer of endoderm 

 cells. 



The tentacles are here solid, consisting of a single row of large 

 endoderm cells encased in a layer of small ectoderm ones, which 

 develop numerous stinging cells. 



Eight so-called " otocysts " (ot.) are found on the inner sirfe-of the 

 bases of certain of the tentacles. Each consists of a sac containing 

 an otolith. The function has long been supposed to be auditory, 

 but Hurst (Nat. Sc., June, 1893) has with much plausibility argued 

 for their true use as organs designed to give warning of approach to 

 an unquiet, wave-disturbed region, in the turmoil of which their frail 

 anatomy might suffer. (The thick umbrella of some contains up to 

 95% to 98% of water !) 



The sexes are separate. In Obelia, the genital glands (gg.) are 

 developed as four masses upon the radial canals, one for each midway 

 between manubrium and margin. The ova after fertilization become 

 ciliated embryos, which soon affix themselves and develop into 

 hydroid colonies. 



It is painful to have to record that these tiny marvels of 

 Nature's excellence of workmanship, are sordidly greedy and vora- 

 cious. The power of the sting-cells has great paralysing action and 

 comparatively large animals are easily captured, and sucked into the 

 mouth. Fig. 6 shows how an Arrow-worm (Sagitta) had been 

 captured and partly doubled in two and so partially swallowed. 

 Copepods, notably Temcra, form another common food. 



Obelia, save in the slight development of the velum of the 

 Medusa, and the form of the latter being rather that of a disc than 

 bell-shaped, is thoroughly typical of the form and development of 

 that division of the Hydroidea where the polypites are borne in 

 cups the Calyptoblastic Hydroidea. 



Endless and complicated modifications exist in other species, 

 and some of these I hope to illustrate and explain before long. 



EXPLANATION OF PLATE IX. 

 Life-History of Obelia geniculata. 



Fig. 1. Natural appearance of the fronds of the Hydroid-stage, as 

 they grow upon the surface of Laminaria. Life size. 



Fig. 2. A single frond of the same much enlarged, pr, perisarc ; 

 c, ccenosarc ; h, hydrotheca ; p, polypite ; b } bud devel- 

 oping to form a polypite later on (note that the perisarc 

 covers the entire surface of the bud ; as growth goes on 



