THE HYDROMEDUSAE 33 



these organs. By a secondary growth they become, in many 

 cases, enclosed by an overgrowth of ectoderm, so that they lie in 

 sacs, which either project on the surface (Trachynema) or are 

 sunk in the mesogloea (Geryonia). 



SEXUAL REPRODUCTION. (No form of asexual reproduction 

 is known among Trachomedusae.) The sexes are separate, the pro- 

 duct of the fertilised ovum is always a medusoid. Segmentation 

 of the ovum is complete ; the endoderm is formed by delamination 

 from the ectoderm. The secretion of mesogloea between ectoderm 

 and endoderm is considerable, except at one pole of the spherical 

 larva, the pole where the mouth is pierced and the tentacles are 

 formed. At this stage the organism presents some resemblance to 

 a hydroid larva, but its conversion to the adult form is achieved 

 by continuous metamorphosis, consisting chiefly in a flattening of 

 its spherical outline and an assumption of the characteristic bell- 

 shape of the adult. The originally simple coelenteron is converted 

 into the canal system of the adult by fusion of the endoderm, 

 except along certain lines, forming an endoderm lamella of the 

 usual type. 



ORDER 4. Narcomedusae. 



DEFINITION. Hydromedusae in which the medusoid form (with 

 one exception) develops directly from the ovum (hypogenesis) ; 

 no alternation of hydroid and medusoid generation occurs. The 

 chief sense organs are tentaculocysts with endodermal otoliths, 

 never enclosed in vesicles. The generative organs lie on the 

 subumbral floor of the gastric cavity or gastric pouches. 



The bell of the MEDUSOID (Figs. 40, 41) is generally flattened, 

 and provided with a strong velum; the mesogloea is thick and 

 extremely tough. The bell is furrowed and its edge incised into 

 a series of lappets, by the peronia, which, as in the Trachomedusae, 

 radiate from the exumbral origins of the tentacles outwards to 

 the circumference, marked by a stripe of cnidoblasts from the 

 marginal ring. The edge of the bell being thus incised, the 

 marginal nerve ring and ring of cnidoblasts are festooned to a greater 

 or less extent round the lappets, instead of forming the unbroken 

 circle which is generally characteristic of the groups already 

 described. 



The four primary tentacles are always placed perradially ; they 

 are retained throughout life (Cunantha), or two of them are 

 dropped (Aeginella), or four interradial tentacles are added 

 (Aegineta) ; many forms however develop more (Solmaris). They 

 are always solid, and are placed in most cases on the exumbrella 

 at some distance from the margin, their endodermal axis penetrat- 

 ing far into the mesogloea ; they retain, however, an endodermal 

 connection with the circular or festoon canal or with the gastric 



