HYDROZOA CRASPEDOTA. 751 



growth. Those of the Pectyllidae terminate in suckers *. The solid ten- 

 tacles have the usual structure : their cnidoblasts are very variously 

 arranged, scattered, gathered into rows, rings, or on a terminal capitulum : 

 ciliated cells and sense-cells furnished with stiff sense-hairs are present as 

 well. The latter in the Trachynemid Marmanemidae may be aggregated 

 in a patch or a circle at the apices of the tentacles, in longitudinal rows 

 along them, or in 'tactile combs' situated in pairs at the bases of the 

 tentacles as well as between them. The ectodermal musculature of the 

 solid tentacles is striated. 



The tentaculocysts are primitively four and interradial, and so persist 

 in Petasus and Dipetasus, but they are usually displaced by the unequal 

 growth of the bell-margin, or the formation of interradial tentacles ; there are 

 usually four radial tentaculocysts as well, and then the eight organs take 

 an intermediate or adradial position. The number is seldom increased ; e. g. 

 Olindias has one to two hundred or more. Among Geryonidae \heLiriopidae 

 have eight, the Carmarinidae twelve, half radial, half interradial, and 

 doubling the number of the radial canals. They are free always at first, 

 and persist in this condition in the family Aglauridae, the Petasid sub-family 

 Petachnidae, and the Trachynemid Pectyllidae, but in the Petasid Olindiadae, 

 the Trachynemid Marmanemidae, and the family Geryonidae they become 

 inclosed by the growth of the ectoderm of the bell-margin in vesicles, 

 which in the last-named are sunk within the exumbrellar mesoglaea. The 

 endodermal axis consists of two to three, rarely four cells, the apical cell 

 dilated, and lodging a single rounded or ovate calcareous otolith. The 

 ectoderm consists in part of auditory sense-cells. Marginal bulbs and 

 cirri (p. 761) are occasionally present. 



The manubrium is very muscular, short when attached to a gastric 

 peduncle (supra], elongate in other instances. The mouth has four lobes, 

 or in the Carmarinidae six, beset with cnidoblasts : it is, as a rule, extremely 

 dilatable, even exceeding the diameter of the bell, e.g. in Liriope, and may 

 then be used as a sucker. There are radial canals, four in Petasidae, as in 

 the Geryonid Liriopidae, eight in others, except the Carmarinidae, where 

 there are six ; they are united by a circumferential or marginal canal. In 

 the Petasid Olindias, the Trachynemid Pectis, and in some Liriopidae and 

 Carmarinidae centripetal radial canals originate from the marginal canal 

 between the radial canals, but fail to reach the base of the manubrium. 

 Their number is variable, but increases with age. The sexual organs are 



1 These tentacles are solid in Pectis and Pectyllis, and arranged in vast numbers along the edge 

 of the btfl : some of them are sessile. In Pectanthis they are aggregated in sixteen bundles, two 

 bundles between each pair of radial canals. Pectanthis was observed by Haeckel alive : it was able 

 to climb up the glass sides of an aquarium by means of its suckers, or to anchor itself with the bell- 

 margin downwards or reversed. See his report on ' Deep Sea Medusae,' Challenger Reports, iv. 

 21, and PI. VIII. 



