THE HYDROMEDUSAE 35 



maridae, the radial, peronial, and festoon canals are suppressed, 

 being represented only by solid cords of endoderm cells. 



The arrangement of the generative organs varies considerably ; 

 they are always developed from the subumbral wall of the 

 coelenteron, but may form either a continuous ring (Solmaris), or 

 radial pouches (Cunina) ; the radii in which they lie are specifically, 

 not generically, characteristic. 



REPRODUCTION. In some cases the development of the medu- 

 soid from the fertilised ovum follows along the lines of a continuous 

 metamorphosis, the diblastula becoming gradually converted into 

 the form of the adult medusa (Aeginopsis mediterranea). In 

 Cunoctantha octonaria the diblastula becomes parasitic on an 

 Anthomedusan (Turritopsis) ; and both it, and buds formed from it, 

 gradually assume the adult form by a continuous metamorphosis. 

 The life-histories of some other forms cannot be said to be as yet 

 fully understood ; in Cunina parasitica the diblastula is parasitic on 

 Geryonia hastata ; its buds become Narcomedusae of a somewhat 

 Solmaridan type, but the planula does not itself develop into a 

 medusa ; there is thus here an apparent alternation of at least 

 two different generations. In Cunina proboscidea a form of asexual 

 reproduction termed sporogony has been described ; neutral 

 amoeboid cells, neither ova nor spermatozoa, wander from the 

 generative organs into the endoderm and mesogloea, and develop 

 into medusae (Metschnikoff, 13; Brooks, 14; Maas, 44). 



ORDER 5. Hydrocorallinae. 



DEFINITION. Colonial metagenetic Hydromedusae with a cal- 

 careous skeleton, into which the gastrozooids and dactylozooids can 

 be retracted. The skeleton is perforated by coenosarcal tubes, on 

 which the gonophores are generally formed. 



The Hydrocorallinae (Moseley, 37) are colonial and trimorphic 

 and secrete without exception a massive (Millepora) or branching 

 (Allopora) calcareous skeleton, the coenenchyme (coenosteum). The 

 relations of this skeleton are best understood by the conception of a 

 branching and anastomosing hydrorhiza, the ectoderm of which 

 secretes, not a horny perisarc, but calcareous trabeculae which fill all 

 the interspaces between the tubes of soft tissue. The surface of the 

 coenenchyme is either pitted with pores of two or more kinds, 

 gastropores and dactylopores, into which the gastrozooids and 

 dactylozooids can be withdrawn (Millepora, Fig. 42), or is produced 

 into spouts (Spinipora) or cups (Stylaster) for the same purpose. 

 The pores may be scattered, or may be arranged in definite systems, 

 in which the dactylozooids are in lines parallel to, and on each 

 side of, a line of gastrozooids (Distichopora), or in circles round 

 the gastrozooids (Stylaster), Fig. 43, b and c. A circular system 



