716 Proceedings of the Royal Irish Academy. 



sile, three to four inches long when fully expanded, spiral 

 when contracted. Each tentacle has a bulbous base, with 

 a distinct ocellus. No marginal vesicles. Velum of moderate 

 width. Manubrium forms a small projection from the summit 

 of the umbrella, and terminates in foiu' rather indistinct lips. 

 From the base of the manubrium three wide canals are sent off, 

 at equal distances ; these gradually contract in diameter, and 

 finally enter the ring canal. The symmetry of the radiating 

 canals is confined to these three primary trunks. Prom their 

 wide proximal ends each sends olf branches, some of which may 

 be traced to the margin, where they Join the ring canal, while 

 others end blindly in the substance of the umbrella. The 

 branches are very irregular in number, length, and direction. 



The generative elements are formed in oval sporsacs, developed 

 one on each of the three primary canals at the spot where the 

 wider base passes into the narrow continuation. The ova 

 remain in the sac until the planula stage is reached. The 

 planula breaks out of the sac and remains attached to the 

 outer wall for some time. Nearly spherical in shape, it never 

 acquires cilia, and possesses little or no power of locomotion. 

 The gastric cavity is fully formed. Further development un- 

 known. Yery abundant. South coast of Ireland." 



Haeckel, in 1879, found a few specimens at Brighton, and 

 noticed the variability of the canal system. In his " System 

 der Medusen " he has placed the species in the genus Bipleu- 

 rosoma, under the name of D. irregulare^ and gives Allman's 

 A. hemisphcerica as a doubtful synonym. The genus contains 

 two other species, JD. typiciim, Boeck, and B. amphithectum, 

 Haeckel. 



Haeckel's definition of the genus Bipleurosoma is as follows : — 



" Cannotidse, with six branched radial canals leaving a 

 bilateral stomach in two opposite groups, three canals in each 

 group, with irregular branches. The main canals and their 

 branches generally enter the ring canal. Six gonads on the 

 undivided part of the main canals near the stomach." 



This description is based on D. amphithectum, and not on 

 D. typicum or A. hemisphcerica. 



D. amphithectum has a bilateral stomach with six main canals. 

 The gonads are on these canals close to the stomach, and the 

 lateral branches of the main canals are between the gonads and 

 the ring canal. 



