CCELENTERATA: HYDROZOA. 101 



swimming organ, the " nectocalyx" or " disc," from the roof of 

 which a single polypite is suspended (fig. 21). The interior of 

 the nectocalyx is often called the " nectosac," and the term 

 " codonostoma " has been proposed to designate the open 



Fig. 21. — Morphology of Medusidae. a A Medusid {T/taumauiias) seen in profile, 

 showing the central polypite, .the radiating and circular gonocalycine canals, the 

 marginal vesicles and tentacles, and the reproductive organs ; d The same viewed 

 from below. The dotted line indicates the margin of the velum. 



mouth of the bell. The margjn of the nectocalyx is produced 

 inwards to form a species of shelf, running round the margin 

 of the mouth of the bell, and termed the " veil " or velum," 

 by the presence of which the nectocalyx is distinguished from 

 the somewhat similar " umbrella " of the Lucernarida. The 

 endodermal lining of the central polypite or "manubrium" 

 (sometimes called the " proboscis ") is prolonged into four 

 radiating canals, which run to the periphery of the nectocalyx, 

 where they are connected by a circular canal which runs round 

 its circumference, the whole constituting the system of the 

 " nectocalycine canals " (formerly called the " chylaqueous 

 canals "). From the circumference of the nectocalyx depend 

 marginal tentacles, which are usually hollow processes, com- 

 posed of both ectoderm and endoderm, and in immediate 

 connection with the canal system. Also round the circum- 

 ference of the nectocalyx are disposed certain " marginal 

 bodies," of which two kinds may be distinguished. Of these 

 the first are termed " vesicles," and consist of rounded sacs 

 lined by epithelium, and containing one or more solid, motion- 

 less concretions — apparently of carboaate of lime — immersed 

 in a transparent fluid. The second class of marginal bodies, 

 variously termed " pigment-spots," "eye-specks," or "ocelli," 

 consists of little aggregations of pigment enclosed in distinct 

 cavities. The "vesicles" are probably rudimentary organs of 

 hearing, and possibly the eye-specks are a rudimentary form of 

 visual apparatus. The oral margin of the pQlypite may be 



