HYDROZOA CRASPEDOTA. 771 



zone round the large central gastrozooid. The medusa of Velella is known as 

 Chrysomitra striata ; it is a quarter of an inch in diameter, has a single ten- 

 tacle, and its generative products are developed in four groups on the manu- 

 brium as in Anthomedusae^ . The female zooid of Physalia is probably de- 

 tached as a medusa. In all other instances the zooid is a medusoid : it has 

 typically four radial canals, a circumferential canal, and an imperforate manu- 

 brium or spadix which bears the sexual products. The Calycophoridae have 

 many ova, the Physophoridae 2 ' a single ovum. The spadix in the latter grows 

 round the ovum, and its cavity becomes reduced to a system of branched 

 canals, sometimes confounded with the radial canals. A velum is present 

 in the sexual zooid of the Diphyozooid (p. 773) but not in other instances 3 . 

 In the Calycophorids Hippopodius and Epibulia ^ Galeolaria) and in Physo- 

 phoridae, the bell itself consists only of three layers, an endodermal lamella 

 bounded by two ectodermal, an outer and inner, and the bell cavity is 

 small. The zooids are numerous in Physalia, Physophoridae, and the Caly- 

 cophorid Hippopodidae, developed in grape-like bunches on the pedicle, or 

 as in Physalia and Physophora extending up the sides, of the hydrocysts, 

 male and female in connection with the same hydrocyst, or as in Agalma 

 rubrum with different hydrocysts. In the Diphyidae they are developed 

 successively one after another on the pedicles of the polypites. The zooids, 

 male and female, are detached in Hippopodius and Epibulia^ or the male 

 only is so, e. g. in Physopkora^ Halistemma, Forskalia, swimming by means 

 of the cilia covering the bell. The sexual zooid in a Diphyozooid dis- 

 charges the sexual products, and is detached when its successor is 

 ready to replace it. The colonies are usually hermaphrodite. Abyla penta- 

 gona and some species of Diphyes, however, are of separate sexes. (5) The 

 hydrophyllia or bracts ( = Deckstiicke), absent in Discoideae, Physalia and 

 Hippopodidae, are protective zooids of a leaf-like character but of various 

 shapes, attached by a short pedicle either to the coenosarc (Physophoridae) 

 or to the pedicles of the polypites (Calycophoridae). They consist of a 

 lamina of mesoglaea covered by ectoderm, in which cnidoblasts are fre- 

 quently to be found at the apex or projecting angles of the hydrophyllium. 

 They contain a central endodermic canal from which, as in Crystallodes 

 (Agalma) rigidum, a process may extend to the lateral angles. (6) Necto- 

 calyces absent in Discoideae, Physalia, Athorybia, and Rhizophysa. They 

 resemble a craspedote Medusa with four radial canals, minus a manu- 

 brium, mouth and tentacles 4 . (7) The pneumatophore or float, an air- 



1 But see Metschnikoff, ' Medusologische Mittheilungen,' Arb. Zool. Inst. Wien, vi. p. 238 (p. 2 

 of paper). The female, according to him, only brings one ovum to maturity. 



2 This term is used in the text to denote the sub-order, not the family. 



3 Chun's genus Lilyopsis is no exception to this statement, for the groups of zooids are not de- 

 tached; see note, p. 773, and SB. Akad. Berlin, 1886, p. 688. 



* The absence of a manubrium is a great peculiarity ; it has been observed by Mereschkowsky 

 as a constant occurrence in certain species of Bougainvillea. See A. N. H. (5), iii. 1879. 



3 D % 



