HYDROZOA CRASPEDOTA. 755 



depths in the sea ; e. g. Pectyllis at 1250 fathoms, Pectis at 1260, Cunarcha 

 Aeginoides at 1675, and Aeginura myosura at 2150. The majority of the 

 Trachomedusae are small, the Geryomdae excepted : the N arc dine dusae are 

 small or of moderate size. The two giants are the Petasid Olindias Sam- 

 baquiensis and the Geryonid Carmaris Goltschii, both of .which attain a 

 diameter of four inches. 



The Hydroidea s. Polypomedusae, the second order of Craspedota, con- 

 tains both non-colonial and colonial hydroids, fixed with the exception of 

 Hydra and Protohydra (?). The hydroid, or in a colony the hydranth, is 

 a purely nutritive zooid, except in Hydra, where it has sexual organs 

 (p. 328, ante). The free sexual zooid, or Medusa, which is frequently 

 arrested in development and consequently sessile, arises by gemmation 

 either from the coenosarc, from a hydranth, blastostyle or Medusa. 



The non-colonial hydroids are the freshwater genera Hydra and 

 Microhydra, the marine Protohydra^ Tiarella, Heterostephanus, Corymor- 

 phidae, and Monocaulus, in all of which a hydrorhiza is absent 1 . In 

 the colonial forms the hydranth is attached directly to the hydrorhiza in 

 Hydrocorallina : by its hydrocope in some instances, e. g. Clavatella^ 

 Hydractinidae : or, as is most usual, by a more or less branched hydro- 

 caulus 2 . A hydrorhiza is sometimes but feebly developed, as in Myrio- 

 thela : but in most instances it is filiform, or reticulate and spreading, 

 giving origin to new zooids or stems 3 . In Hydractinia and Podocoryne it 



it which makes its position doubtful, (i) It has organs at the base of the velum on its exumbrellar 

 aspect, which appear to be auditory though they possess no otoliths. They are said by Professor 

 Lankester to be tentaculocysts, but his description and figures do not place the matter quite beyond 

 doubt. (2) It has floating embryoes, and the bell-cavity seems to be closed at first, a mode of origin 

 not found in any Trachymedusan, so far as is known, certainly not in Liriope and Geryonia, a point, 

 incontestably established by the recent researches of Brooks and Metschnikoff. As the female has 

 never been observed, the origin of the embryoes from ova is not certain. (3) It is possible that the 

 small columnar Hydroid, devoid of tentacles and enveloped in a tube of mud, found by Mr. A. G. 

 Bourne on the roots of a Pontoderia growing in the tank at the gardens of the Royal Botanic Society, 

 Regent's Park the place where Limnocodium appeared may belong to the life-history of the 

 Medusa. If so, it is the solitary instance of a fixed hydroid stage among Trachytnedtisae met with 

 up to the present time. 



1 The hydrocope of Hydra, Protohydra, and Tiarella ends in a disc ; of Heterostephanus, the 

 Corymorphidae and Monocauhis in a pointed extremity, but it is furnished in the two last-named 

 with long extensile filamentous processes which probably anchor it in the mud or sand in which it is 

 immersed. 



2 In some species of Etfdendrium the main stem and origins of the branches may be fascicled, 

 i. e. consist of a number of separate but closely apposed coenosarcal tubes contained each in their 

 own perisarc : see Hincks, British Hydroid Zoophytes, i, and Allman, Gymnoblastic Hydroids, 

 under the genus. Corydendrium has the same peculiarity, and the further one that from the mode 

 of growth of the colony, 2-3 perisarcal tubes may be inclosed in a common perisarcal tube : see 

 Weismann, Entstehung der Sexualzellen bei den Hydromedusen, Jena, pp. 35-6. The erect growing 

 species of the genus Lafoea have the stem composed of several tubes. Anisicola Halecioides has a 

 second stem accompanying the hydranth-bearing stem, which itself bears no hydranths, but is con- 

 nected to the primary stem at short intervals : see Jickeli, M. J. viii. p. 636. 



3 In Coppinia the thecae for the hydranths and blastostyles are closely packed, hiding the 



3 C 2 



