CCELENTERATA : HYDROZOA. 



1 II 



The hydrosoma of one of the Discophora ( = a Gymnoph- 

 thalmate Medusa) is composed of a single, gelatinous, bell-shaped 

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

 which a single polypite is suspended (fig. 30). The interior of 

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

 " codonostoma " has been proposed to designate the open 



Fi.?. 30. Morphology of Meclusidae. a A Medusid (Tkaumantias) seen in profile, 

 showing the central polypite, the radiating and circular gonocalycine canal, the 

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

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



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

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

 of the mouth of the bell, and termedthe " veil " or "velum," by 

 the presence of which the nectocalyx is distinguished from the 

 somewhat similar "umbrella" of the Lucernarida. The en- 

 dodermal lining of the central polypite or " manubrium " (some- 

 times called the " proboscis ") is prolonged into four, sometimes 

 more, radiating canals, which run to the periphery of the nec- 

 tocalyx, 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 carbonate 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 



