40 R. T. Günther 



employcd, l)ut I should like to propose the more definitely descrip- 

 tive luime oi citidopliars for tbem as well as for kindred structuresi. 

 The bases of the tentacles are much swollen and in some eases 

 seemed to be providcd witb a pigmented patch whieh might be 

 cousidered homologous witb the ocellus of the Cladonemid genera. 



The Velum and Subumbrella. 



The velum notwithstanding tbe sessile habit of the animai is 

 well developed, and so is the musculature of the subumln-ella. 

 When attached to the PhylUrhoe the Mnestra makes repeated swim- 

 ming movements l)y contracting the velar and subumbrella circular 

 muscles; and even after Separation from the host feeble spasmodic 

 contractions will continue for some time, but owing partly to the 

 small volume of the shallow subumbrella cavity, partly to the want 

 of vigour in the muscular contractions, the eflbrts are insufficient 

 for actual locomotion, even if the vigour of the contraction be 

 artificially increased by Stimulation. 



The most probable explanation of the retention of the velar 

 swimmiug muscles in a state of efticient contractility seems to be 

 that they are required for some purpose other thau that of locomotion. 

 Although not admitting of absolute proof, there is ground for a 

 reasonable conclusion that these muscles by their contraction, and the 

 umbrella jelly by its elasticity, increase and diminish the size of the 

 gastric Spaces. And thus a source of power commonly employcd 

 l>y medusae for swimming, seems to have become applied to a 

 suctorial mechanism in Mnestra where organs of propulsion would 

 bc useless. In Illustration of another case in which the alimentary 

 System has been strengthened by the acquisition of an organ from 

 the locomotor system, we may mention those Crustacea in the life 

 history of which swimming appendages turn into masticatory organs. 



The Gastrovascular System. 



The gastrovascular system in its most typical condition is 

 simple. There is a central stomach, four radial canals and a cir- 

 cular. canal. The manubrium is invariably inserted into a pit ou 



* These structiires must not be confounded with the nematophores of the 

 Phiniulaiidne. 



