n CNIDARIA SUBGENITAL CAVITIES 101 



shown by the jelly which functions as supporting tissue in the Cfcno- 

 /i/i/ir/f, containing, as it does, nerve, muscle, and connective tissue 

 elements. All these elements are usually represented by more or less 

 branched fibres. 



As to the origin of the various mesodermal supporting formations, we have the 

 following remarks to make. The homogeneous supporting membrane of the Hydroids, 

 and the corresponding jelly substance of the Mcduscc, Corals, and Ctcnophora is a 

 product of secretion deposited between the ectoderm and endoderm. Whether both 

 layers take part in this secretion or only one, and if so which, is a question difficult 

 to decide. The cell elements which lie in the jelly, on the contrary, seem almost 

 exclusively to arise by the immigration of cells from the ectoderm. In Corals 

 the ectoderm soon becomes multi-laminar. The cells of the deeper layers become 

 mesodermal connective tissue cells by the rise between them of homogeneous sub- 

 stance. Many observers, therefore, consider the Coral mesoderm as only a more 

 deeply lying portion of the ectoderm. 



IX. Funnel Cavities (Septal Funnels) ; Subgenital Cavities, 

 Subgenital Chamber. 



These formations, met with in many Acmspeda (analogous forma- 

 tions are very rarely found in Craspedotci) are in the lower forms 

 represented by 4 interradial funnel-shaped depressions of the sub- 

 umbrella round the oesophagus (Fig. 99, H, I, st, st', p. 130). They 

 project more or less far into the gastric cavity, within the septa which 

 separate the 4 gastric pouches. In Discomedusce they become 4 some- 

 what flat interradial subgenital cavities. Their roof is close to the 

 subumbrellar wall of the central gastric cavity in which the genital 

 organs develop. The membrane which separates the two cavities thus 

 becomes the genital membrane. In two families of the Rhizoxtomce, 

 (the Venuridce and Cmmbessidce\ the 4 sub-genital cavities unite in 

 the centre of the umbrella to form a spacious sub-genital chamber 

 (portions siibgenitalis, Fig. 70 D, sgp, p. 85), which opens outward by 

 four interradial apertures through the subumbrella into the umbrella 

 cavity. The dorsal roof of the chamber is formed by the gastro-genital 

 membrane, which separates it from the gastric cavity above it. The 

 subgenital chamber separates the central gastric cavity from the 

 oesophagus. The two remain connected only by the 4 canals which 

 run perradially (pfk}. 



X. The Sexual Organs. 



The Cnidaria are either sexually separate, like the Hydrozoa and the 

 Scypliozofi (with a few exceptions, e.g. Hydra and a few CladonemiJn', 

 Cerianthiis, Chrysaora), or hermaphrodite like the Ctenophora. 



In the colonial Cnidaria we find male and female individuals either 

 in the same stock (monoecious), and this is the rule ; or on different 

 stocks (dioecious). 



