122 THE NATURE AND VARIETIES OF 



2. That in so natural a group, tlie relative position as- 

 sumed for the puny bare-eyed Medusae must hold also for 

 their portly brethren of the hood-eyed kind. 



A few details may here be given of the declension re- 

 ferred to, from fi'ee Medusae to germ-sacs of so simple a 

 kind, that they might fairly be termed mere tunicated ova. 

 The transition is so gradual, that Prof Huxley considers it 

 impossible to draw any definite line of distinction between 

 true Medusoids and mere proliferous cysts, or sporosacs, as 

 they are termed by Prof. Allman.* The distinction, how- 

 ever, is a convenient one, if not strictly philosophical. A 

 Medusoid is a gemma, usually detached as a free zooid, and 

 consisting of a bell-shaped mantle or umbrella, fringed with 

 tentacles at its margin, and having a cylindrical process 

 (manubrium) depending from the centre of its concavity. 

 The manubrium in its most complete form is itself a polype- 

 like organ, sometimes tentaculated at its free extremity. 

 The umbrella may be regarded as a hemispherical swim- 

 ming disc developed, round the base of the central struc- 

 ture, after the prevailing type of arrangement in these ani- 

 mals. In its full development it contains canals in its sub- 

 stance radiating from the central or gastric cavity of the 

 manubrium, and opening into a circular canal at the free 

 margin of the bell. The ovaries are usually situated at the 

 points of origin of these canals. The umbrella in some 

 species is converted by the cohesion of its edges into a shut 

 sac, which serves as a marsupium or uterus for the nur- 

 ture of the ova after impregnation. In the sporosac, which 

 is never detached as a free zooid, the manubrium is gene- 

 rally represented by a central imperforate columella, and 



* Huxley's Oceanic Hydrozoa (Ray Soc), p. 137. AUman's terms are 

 meduscB and sporosacs — tlie last lias an unfortunate similarity to Filippi's 

 term Sporocyst, applied to tlie gemmiparous tubes of the Trematoda, 

 structures of a totally different import. 



