122 THE VOYAGE OF H.M.S. CHALLENGER. 



organs possess essentially the same structure as in the preceding Eudoxidse. The bract 

 occurs only in two forms, one of which (Lilsea) is equal to that of Praya, or Lilyopsis ; 

 it is rounded and has a phyllocyst with four radial canals ; the other (Ersasa) is 

 similar to that of Muggisea or Diphyes; it is pointed, cucullate or spathiform, and has 

 a simple phyllocyst, without radial canals. 



Gonophores. — The sexual medusomes of the ErsEeidse have the same form and structure 

 as those of the Eudoxidse ; and mainly of those species, where two or more gonophores 

 are produced in a single cormidium (compare above, pp. 100 and 121). 



The special nectophore, which is characteristic of the Ersaeidse, and distinguishes 

 them from the Eudoxidse, has been originally the first gonophore. This has completely 

 lost its manubrium, and has, therefore, given up its sexual function ; it has been 

 developed merely as an organ of swimming. 



Ontogeny. — On the development of the Ersseidae, compare above, pp. 100-102. 



Synopsis of the Genera of Ersseidae. 



Bract spathiform, with an apex. Phyllocyst simple, without radial canals, . . . 17a. Ersxa. 



Bract rounded, without apex. Phyllocyst with four radial canals, . . . .17b. Lilsea. 



Genus 17a. Erssea?- Eschscholtz, 1829. 

 Erssea, Eschscholtz, System der Acalephen, p. 127. 



Definition. — Ersseidae with a campanulate or spathiform bract, divided by a collar 

 constriction into a proximal cowl, with an apical point, and a distal mantle, with a wide 

 ventral fissure. Phyllocyst large, simple, without radial canals. (Cormidia of the genus 

 Diphyopsis.) 



The genus Erssea was founded by Eschscholtz, in 1829, to include two Atlantic 

 monogastric Diphyidae, differing from Eudoxia in the possession of a special nectophore, 

 which is prominent from the cavity of the bract (" Tubulus suctorius unicus ; pars 

 corporis nutritoria cavitate parva natatoria, tubuli instar prominenti instructa"). The 

 two species, however, which Eschscholtz there described [Erssea quoyi and Erssea 

 gaimardi), 2 are too incompletely represented to determine with certainty their position 

 and their connection with any species of Diphyopsis. Possibly Erssea gaimardi may 

 be the free cormidium of Diphyopsis campanulifera, and Erssea bojani ( = Eudoxia 

 hojani, Eschscholtz), the cormidium of Diphyopsis disp>ar. I myself observed in 1866, 

 in the Canary Island Lanzerote, numerous free-swimming specimens of the species figured in 

 PI. XXXIV. as Erssea compressa, and afterwards recognised its ontogenetic connection 

 with Dij)hyopsis compressa (PI. XXXIII.). 



1 Erssea = f{<7«?«, covered with dew. - 1, p. 128, Taf. xii. figs. 3, 4. 



