COELENTERATA. 



117 



(hence Steganophfhalmata, Fig. 103). In some Lucernaridae the marginal bodies 

 are present as the so-called marginal anchors (HaUclystus, Fig. 134). There are 

 eight of them, four being called radial in position, and the other four interradial ; 

 they are placed respectively between the eight marginal lobes. They end in a 

 small swelling with thread cells ; their middle portion is surrounded by a kind 

 of collar formed of sticky glandular ectoderm cells, and on the sub-umbrella side 

 of their basal part is an eye -spot. 



In other Acraspeda the endoderm at the end of the sense tentacle is thickened 

 and loaded with concretions, and the ectoderm round its base is columnar and 

 provided with cilia and sense hairs (Fig. 103). The fibrillar continuations of 

 these cells form a nervous network, with ganglion cells, round the base of the 

 tentacle (Fig. 103, F). 



Visual organs are sometimes present in 

 this sensory epithelium. They are most 

 complicated in Charybdca, where there 

 are two large terminal eyes with a retina, 

 vitreous humour, lens, and cornea. 



FIG. 104. Sense organ on the 

 nerve-ring and circular vessel 

 of Octorchis (after the Hert- 

 wigs). Rb Sense organ ; 0, 

 two otoliths ; Hit, auditory 

 hairs ; Hz auditory cells ; Nv 

 upper nerve-ring ; Eg circular 

 vessel (type of Vcsiculata). 



Fio. 105. Tentaculocyst of Geryo- 

 n in (after O. and R. Hertwig). 

 N, N' the auditory nerves ; Ot 

 otolith ; Hz auditory cells ; Hh 

 auditory hairs (type of the Traeho- 

 medMsae). 



Reproductive organs. The medusae are almost always dioecious, 

 only in rare cases are they hermaphrodite (Chri/saora). 



The sexual cells of the Craspedota appear always to arise in ectoderm, 

 and those of the Acraspeda in endoderm. In the Antliomedusae they 

 are developed in the ectoderm of the manubrimn (Fig. 95) in four 

 radial streaks, each of which later splits into two. In the other 

 Craspedota (Leptomedusae) they are placed in the ectoderm of the 

 sub-umbrella beneath the radial canals (Fig. 106). It appears, how- 

 " <r ever, that in many medusae (Obelia) the generative cells arise in the 

 ectoderm of the manubrium, and then later migrate to the ecto- 

 derm below the radial canals, where they occasionally Avander into 

 the endoderm and ripen partly in the ectoderm and partly in the 

 endoderm. 



In the Acraspeda the sexual cells arise in the endodermal walls of 



