Class I. Hydrozoa. Order 1. Hydromedusce. 



105 



■cells. If the colony be disttu-bed, these individuals roll themselves up in a spiral 

 fashion and then uncoil again. The tentacles are absent from others, which 

 seem then to function only as the tactile apparatus, whilst in the former case 

 they may perhaps be regarded as defensive polyps (dactylozooids). 



The mediisoid generation formed by budding from the 

 polypoid generation, usually attains only a small size in this Order. 

 Round the edge of the disc runs a thin collar-like, horizontal inturned 

 ledge, the velum, which narrows the opening of the bell (hence 

 the name Craspedota] .* The edge of the umbrella is entire and 

 bears the naked sense organs, either auditory or optic (rarely 



1': Fig. 61. Various forms of the sexual generation of Hydromedusoids. Dia- 

 grammatic longitudinal sections. A free living medusa. B sessile, relatively little modified 

 medusa. — D more modified forms. E — P the most degenerate form. E without umbrella. 

 Ji' a simple knob-like outgrowth from the polyp, g sexual cells, m mouth, r radial canal, 

 t tentacle, v velum. — Orig. 



both in the same individual). There is usually a small number of 

 simple radial canals (4 — 8) in the umbrella, connected by a 

 marginal ring canal. Ova and spermatozoa develop in these 

 ■canals, or on the outer wall of the manubrium; they arise in some 

 forms from the ectoderm, in others from the endoderm; in 

 some species, the ova appear in the endoderm, the spermatozoa in 

 the ectoderm, or vice versa. 



* KpaiTTreSoi', a border. 



